Enter the start date and time and the days, hours, minutes and seconds to add or subtract. You get both the later and the earlier date and time, with the day of the week.
Amount fields you do not need can be left blank (or 0). A blank start time counts as 0 (midnight, 12:00:00 AM). Hours and minutes can be entered as large numbers, such as 36 hours or 90 minutes.
Result
Enter a start date and time and the amount of time to add in the fields on the left and press "Calculate". The result will appear here.
Table of Contents
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What you can do on this page
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What is this calculation used for?
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How to Use
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Formula
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Symbols and terms
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Good to know before you start
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How to calculate it in Excel
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How to calculate it in Google Sheets
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How to calculate it in Python
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How to write it in LaTeX and other math languages (copy and paste)
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How to have ChatGPT do the calculation
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DataChef Features
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Related Features
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NumberChef Calculators List
What you can do on this page
- Add days, hours, minutes and seconds to a start date and time, or subtract them, and get both the later and the earlier date and time with the day of the week
- Answers questions that cross midnight, such as "What day and time is 36 hours from now?" or "When was 48 hours ago?"
- Handles crossing midnight, the end of the year and February 29 in leap years correctly, just as on a calendar
- A plain-language explanation of how clock time carries over (60 seconds = 1 minute, 60 minutes = 1 hour, 24 hours = 1 day) and copy-and-paste formulas for Excel, Google Sheets and Python are all on this page
This page is for calculations that include a time of day. If you only need a date "some days or months later" without a time, use the date calculator for adding and subtracting days among the related pages.
What is this calculation used for?
Finding when a night shift ends (shifts that cross midnight)
An 8-hour shift that starts at 9 PM ends at 5 AM the next morning (in 24-hour time, 21 + 8 = 29, and the part past 24 carries into the next day: 29 − 24 = 5). With a 1-hour break, 9 hours later is 6 AM.
In nursing, caregiving, factories, stores and other jobs that run through the night, "When do I finish?" and "When can I nap, and for how long?" are exactly this calculation, used every day to plan shifts.
Keeping the right time between doses (every 6 or 12 hours)
If you take a medicine that says "at least 6 hours between doses" at 8:30 AM, the next dose can be taken at 2:30 PM or later. For a medicine taken every 12 hours, the dose after 8:30 AM is at 8:30 PM.
With fever reducers and antibiotics, the time between doses matters for both how well they work and safety, so it helps to know the exact time of the next dose. Always follow your doctor's or pharmacist's directions for the actual timing and number of doses.
Turning "within 48 hours" or "within 72 hours" into a date and time
Deadlines are often given in hours, such as "pay within 48 hours after the auction ends" or "free cancellation within 72 hours of booking". Within 48 hours from Friday at 3 PM means the deadline is Sunday at 3 PM.
If you think "2 days later" and count only the dates, you can misread the deadline by the time of day. For a deadline given in hours, work out the exact date and time as on this page, and you will be sure.
Timing cooking and fermenting (sous vide, bread, yogurt)
Yogurt started at 9 PM is ready 8 hours later, at 5 AM the next morning, and a 90-minute first rise for bread started at 2 PM ends at 3:30 PM.
For long cooking such as sous vide, smoking or pickling, this calculation helps not only with "When will it be ready?" but also with working backward: "To eat at 7 AM, when do I need to start?"
Checking the arrival time of an overnight bus or a flight
An overnight bus that leaves at 10:05 PM and takes 7 hours 35 minutes arrives at 5:40 AM the next morning (minutes: 5 + 35 = 40; hours in 24-hour time: 22 + 7 = 29, which passes midnight and becomes 5 AM).
Knowing the arrival time from the departure time and the travel time makes it easier to plan what comes after. On a flight to another time zone, this calculation gives the arrival in the departure city's time, so to get the local time you also need to add or subtract the time difference.
Formula
The date and time some days and hours later or earlier
Standard notation
(the usual math form)
\(T_1\)
\(=\)
\(T_0\)
\(+\)
\(D\)
\(T_2\)
\(=\)
\(T_0\)
\(-\)
\(D\)
In words
(symbols replaced with words)
③
\(T_1\): later date and time
\(=\)
①
\(T_0\): start date and time
\(+\)
②
\(D\): amount of time
\(T_2\): earlier date and time
\(=\)
\(T_0\): start date and time
\(-\)
\(D\): amount of time
The formula in words
①
Take the
\(T_0\): start date and time
②
add the
\(D\): amount of time (days, hours, minutes, seconds)
③
and you get the
\(T_1\): later date and time
(subtract instead, and you get the earlier date and time, \(T_2\))
Quick example
For "1 day and 2 hours after July 12, 2026, 8:30 AM", add 1 to the day, 12, and 2 to the hour, 8
later date and time (July 13, 10:30 AM)
\(=\)
start date and time (July 12, 8:30 AM)
\(+\)
amount of time (1 day and 2 hours)
\(12 + 1 = 13\)
\(8 + 2 = 10\)
Key idea
Add and subtract days, hours, minutes and seconds unit by unit. If a sum goes past 60 seconds, 60 minutes or 24 hours, carry it into the next larger unit (for example, 50 minutes + 25 minutes = 75 minutes → 1 hour 15 minutes). If a difference goes below zero, borrow from the next larger unit (for example, 1 hour 30 minutes before 12:15 AM → 10:45 PM the day before).
The results of this calculator already include carrying and borrowing, as well as crossing into another month or year and leap years (February 29).
Putting the amount of time into seconds
Standard notation
(the usual math form)
\(S\)
\(=\)
\(86400\)
\(\times\)
\(d\)
\(+\)
\(3600\)
\(\times\)
\(h\)
\(+\)
\(60\)
\(\times\)
\(m\)
\(+\)
\(s\)
In words
(symbols replaced with words)
⑧
\(S\): total in seconds
\(=\)
②
\(86400\): seconds in a day
\(\times\)
①
\(d\): days
\(+\)
④
\(3600\): seconds in an hour
\(\times\)
③
\(h\): hours
\(+\)
⑥
\(60\): seconds in a minute
\(\times\)
⑤
\(m\): minutes
\(+\)
⑦
\(s\): seconds
The formula in words
①
Take the
\(d\): days
②
and multiply by
\(86400\): seconds in a day
③
take the
\(h\): hours
④
and multiply by
\(3600\): seconds in an hour
⑤
take the
\(m\): minutes
⑥
and multiply by
\(60\): seconds in a minute
⑦
add all of these and the
\(s\): seconds
⑧
and you get the
\(S\): total in seconds
Quick example
"1 day and 2 hours" in seconds is
total in seconds (93600 s)
\(=\)
seconds in a day (86400)
\(\times\)
days (1 day)
\(+\)
seconds in an hour (3600)
\(\times\)
hours (2 hours)
\(S = 86400 \times 1 + 3600 \times 2 = 93600\)
Key idea
1 day = 24 hours, 1 hour = 60 minutes and 1 minute = 60 seconds, so a day has \(24 \times 60 \times 60 = 86400\) seconds and an hour has \(60 \times 60 = 3600\) seconds.
An amount in mixed units, such as "1 day and 2 hours", becomes a single addition once everything is in the smallest unit, seconds. Then it is easy to compare or add. The "Step by step" part of this calculator's result shows the total in seconds found with this formula.
Turning hours into days and hours (carrying at 24)
Standard notation
(the usual math form)
\(h\)
\(=\)
\(24\)
\(\times\)
\(q\)
\(+\)
\(r\)
In words
(symbols replaced with words)
④
\(h\): total hours
\(=\)
①
\(24\): hours in a day
\(\times\)
②
\(q\): days
\(+\)
③
\(r\): hours left over
The formula in words
①
Take
\(24\): hours in a day
②
multiply by the
\(q\): days
③
add the
\(r\): hours left over
④
and you get the
\(h\): total hours
(to find them, divide the total hours \(h\) by \(24\). The whole-number answer is the days \(q\), and the remainder is the hours left over \(r\))
Quick example
To see how many days and hours "36 hours later" is, divide 36 by 24 and use the whole-number answer and the remainder
total hours (36 hours)
\(=\)
hours in a day (24)
\(\times\)
days (1 day)
\(+\)
hours left over (12 hours)
\(36 = 24 \times 1 + 12\)
Key idea
36 hours later is the same as "1 day and 12 hours later". For 36 hours after 8:30 AM, move 1 day ahead to 8:30 AM the next day, then 12 more hours to 8:30 PM the next day.
Minutes and seconds work the same way: carry at 60 minutes = 1 hour and 60 seconds = 1 minute (for example, 90 minutes = 1 hour 30 minutes). You can enter large numbers such as "36 hours" or "90 minutes" into this calculator as they are. It carries them over into the date and time for you.
The basics of adding time to a date and time: add or subtract days, hours, minutes and seconds unit by unit, and carry or borrow at 60 seconds = 1 minute, 60 minutes = 1 hour and 24 hours = 1 day. An amount in mixed units becomes a single addition once you put it all into seconds.
Symbols and terms
Symbols
| \(T_0\) | T sub 0 | The start date and time (the point you count from). The small 0 at the lower right is a subscript meaning "number 0 = the starting point". |
| \(T_1\), \(T_2\) | T sub 1, T sub 2 | The resulting dates and times. \(T_1\) is the later one, and \(T_2\) is the earlier one. |
| \(D\) | D (capital) | The amount of time to add or subtract, with days, hours, minutes and seconds taken together. (Example - "1 day and 2 hours") |
| \(d\), \(h\), \(m\), \(s\) | d, h, m, s | The parts of the amount of time: days (\(d\)), hours (\(h\)), minutes (\(m\)) and seconds (\(s\)). For "1 day and 2 hours", \(d = 1\) and \(h = 2\). They are the first letters of day, hour, minute and second. |
| \(S\) | S (capital) | The amount of time written in seconds. Found with \(S = 86400d + 3600h + 60m + s\). |
| \(q\), \(r\) | q, r | When the total hours are divided by 24, the whole-number answer (the days \(q\)) and the remainder (the hours left over \(r\)). For \(36 \div 24 = 1\) remainder \(12\), \(q = 1\) and \(r = 12\). |
Terms
| 24-hour time | A way of writing the time with the hours 0 to 23 and no AM or PM. 8 PM is 20:00. In the US it is often called military time. This calculator takes the start hour either with AM or PM or in 24-hour time, and shows the results with AM and PM. |
| base 60 | A way of counting where 60 makes one of the next larger unit (also called sexagesimal). 60 seconds carry to 1 minute, and 60 minutes carry to 1 hour. Note that between hours and days, you carry at 24 instead. |
| carrying | In addition, moving the part that goes past the limit of a unit into the next larger unit (50 minutes + 25 minutes = 75 minutes → 1 hour 15 minutes). With time, you carry at 60 or 24. |
| borrowing | In subtraction, taking what you are short from the next larger unit (1 hour 30 minutes before 12:15 AM → 10:45 PM the day before). With time, you borrow 60 or 24. |
| leap year | A year with a February 29. It usually comes every 4 years, but years divisible by 100 are common years unless they are divisible by 400 (2100 is not a leap year, 2000 was). When a calculation crosses the end of February, whether it is a leap year changes the resulting date by 1 day. |
| weekly cycle | The days of the week repeat every 7 days. Adding an exact multiple of 7 days (for example, 168 hours = 7 days) lands on the same weekday as the start. |
Good to know before you start
Here is what helps you use the calculation on this page with real understanding, not just by pressing the button.
If you get stuck, going back over these topics is the quickest way forward.
| Time of day and elapsed time (Grades 2–3) |
|
| 24-hour time (Grade 3 and up) |
|
| Adding and subtracting with regrouping (Grade 2) |
|
| Division with remainders (Grade 4) |
|
| How the calendar works (Grades 3–4) |
|
How to calculate it in Excel
Copy the whole table below and paste it into cell A1 in Excel. It works as is.
Table for the date and time some days and hours later or earlier
| Start date | 2026-07-12 |
| Start time | 8:30:00 |
| Days | 1 |
| Hours | 2 |
| Minutes | 0 |
| Seconds | 0 |
| Later date and time | =B1+B2+B3+B4/24+B5/1440+B6/86400 |
| Earlier date and time | =B1+B2-B3-B4/24-B5/1440-B6/86400 |
| Weekday of the later date | =TEXT(B7,"dddd") |
Table to put the amount of time into seconds
| Days | 1 |
| Hours | 2 |
| Minutes | 0 |
| Seconds | 0 |
| Total in seconds | =B1*86400+B2*3600+B3*60+B4 |
Table to turn hours into days and hours
| Total hours | 36 |
| Days (quotient) | =QUOTIENT(B1,24) |
| Hours left over (remainder) | =MOD(B1,24) |
Inside Excel, dates are running numbers with 1 day = 1 (serial numbers), and a time of day is a decimal: the fraction of a day. So after combining "start date + start time" into one date and time, add or subtract days as they are, hours divided by 24, minutes divided by 1440 and seconds divided by 86400 (B7 and B8 in the first table are this formula). Type the start date in the form 2026-07-12, which Excel reads as a date whatever your regional settings are.
If B7 and B8 show up as numbers such as "46216.4375", change the cell format to the custom format "m/d/yyyy h:mm:ss AM/PM". In the first table, the later date and time is 7/13/2026 10:30:00 AM and the earlier one is 7/11/2026 6:30:00 AM. =TEXT(B7,"dddd") in B9 shows the weekday of the later date (Monday).
The second table gives 93600 (seconds), and the third gives 1 day and 12 hours left over. Just replace column B with your own numbers.
If B7 and B8 show up as numbers such as "46216.4375", change the cell format to the custom format "m/d/yyyy h:mm:ss AM/PM". In the first table, the later date and time is 7/13/2026 10:30:00 AM and the earlier one is 7/11/2026 6:30:00 AM. =TEXT(B7,"dddd") in B9 shows the weekday of the later date (Monday).
The second table gives 93600 (seconds), and the third gives 1 day and 12 hours left over. Just replace column B with your own numbers.
How to calculate it in Google Sheets
Copy the whole table below and paste it into cell A1 in Google Sheets. It works as is.
Table for the date and time some days and hours later or earlier
| Start date | 2026-07-12 |
| Start time | 8:30:00 |
| Days | 1 |
| Hours | 2 |
| Minutes | 0 |
| Seconds | 0 |
| Later date and time | =B1+B2+B3+B4/24+B5/1440+B6/86400 |
| Earlier date and time | =B1+B2-B3-B4/24-B5/1440-B6/86400 |
| Weekday of the later date | =TEXT(B7,"dddd") |
Table to put the amount of time into seconds
| Days | 1 |
| Hours | 2 |
| Minutes | 0 |
| Seconds | 0 |
| Total in seconds | =B1*86400+B2*3600+B3*60+B4 |
Table to turn hours into days and hours
| Total hours | 36 |
| Days (quotient) | =QUOTIENT(B1,24) |
| Hours left over (remainder) | =MOD(B1,24) |
In Google Sheets, the adding and subtracting formulas, QUOTIENT and MOD work exactly as in Excel, and =TEXT(B7,"dddd") shows the weekday the same way. Copy the whole table, paste it into cell A1, and replace column B with your own numbers.
If a result shows up as a number (a serial number), change it with Format > Number > Date time in the menu.
If a result shows up as a number (a serial number), change it with Format > Number > Date time in the menu.
How to calculate it in Python
from datetime import datetime, timedelta
start = datetime(2026, 7, 12, 8, 30, 0) # start date and time (year, month, day, hour, minute, second)
days, hours, minutes, seconds = 1, 2, 0, 0 # amount of time to add or subtract
difference = timedelta(days=days, hours=hours, minutes=minutes, seconds=seconds)
after = start + difference # later date and time
before = start - difference # earlier date and time
weekday_names = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']
print(f"Total in seconds: {difference.total_seconds():.0f} seconds")
print(f"Later: {weekday_names[after.weekday()]}, {after:%B %d, %Y, %I:%M:%S %p}")
print(f"Earlier: {weekday_names[before.weekday()]}, {before:%B %d, %Y, %I:%M:%S %p}")
Runs with the standard library only. timedelta is an object for a length of time, and you can pass it days, hours, minutes and seconds together. Just add or subtract a timedelta to a datetime, and Python does all the carrying and borrowing (60 minutes = 1 hour, 24 hours = 1 day), as well as crossing months and years and leap years, just as on a calendar.
Change the start date and time and the amount of time at the top, then run it.
How to write it in LaTeX and other math languages (copy and paste)
The date and time some days and hours later or earlier
T₁ = T₀ + D, T₂ = T₀ − D
T_1 = T_0 + D, \quad T_2 = T_0 - D
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>
<msub><mi>T</mi><mn>1</mn></msub><mo>=</mo>
<msub><mi>T</mi><mn>0</mn></msub><mo>+</mo><mi>D</mi>
<mo>,</mo><mspace width="0.7em"/>
<msub><mi>T</mi><mn>2</mn></msub><mo>=</mo>
<msub><mi>T</mi><mn>0</mn></msub><mo>−</mo><mi>D</mi>
</mrow>
</math>
T_1 = T_0 + D, T_2 = T_0 - D
after = DatePlus[start, {{d, "Day"}, {h, "Hour"}, {m, "Minute"}, {s, "Second"}}]
T1 := T0 + dur; T2 := T0 - dur; # D is reserved in Maple (the differential operator), so dur is used
after = start + days(d) + hours(h) + minutes(m) + seconds(s);
T_1 = T_0 + D, T_2 = T_0 - D
Putting the amount of time into seconds
S = 86400d + 3600h + 60m + s
S = 86400d + 3600h + 60m + s
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>
<mi>S</mi><mo>=</mo>
<mn>86400</mn><mi>d</mi>
<mo>+</mo><mn>3600</mn><mi>h</mi>
<mo>+</mo><mn>60</mn><mi>m</mi>
<mo>+</mo><mi>s</mi>
</mrow>
</math>
S = 86400d + 3600h + 60m + s
totalSeconds = 86400*d + 3600*h + 60*m + s
S := 86400*d + 3600*h + 60*m + s;
S = 86400*d + 3600*h + 60*m + s;
S = 86400d + 3600h + 60m + s
Turning hours into days and hours (carrying at 24)
h = 24q + r
h = 24q + r
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>
<mi>h</mi><mo>=</mo>
<mn>24</mn><mi>q</mi>
<mo>+</mo><mi>r</mi>
</mrow>
</math>
h = 24q + r
q = Quotient[h, 24]; r = Mod[h, 24]
q := iquo(h, 24); r := irem(h, 24);
q = floor(h/24); r = mod(h, 24);
h = 24q + r
How to have ChatGPT do the calculation
You are a date and time calculation assistant. Do the following calculation by actually running Python code, and base your answer only on the dates and times from the execution result (do not answer by mental math or guessing). Find the date and time you get by adding "1 day and 2 hours" to July 12, 2026, 8:30:00 AM, and the date and time you get by subtracting it. Carry and borrow at 60 seconds = 1 minute, 60 minutes = 1 hour and 24 hours = 1 day, and handle crossing months and years and leap years just as on a calendar. For each, show the code you used and the resulting date, time (with AM or PM) and day of the week.
How to Use
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1Enter your numbersType the numbers you want to calculate with into the input fields
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2CalculatePress the "Calculate" button
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3Check the resultThe result appears on the spot. The same page also explains the idea behind the calculation and the formula
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