How many hours between 9:30 AM and 5:15 PM?

7 hours 45 minutes, or 7.75 decimal hours, or 27,900 seconds. Use the "Between Two Times" mode with start time 09:30 and end time 17:15 (24-hour format). If both are on the same day, the date fields default to today so you can leave them alone.

Can I calculate duration across midnight or across multiple days?

Yes - just set the end date to the later day. For example, "from 11 PM Friday to 7 AM Saturday" - put 23:00 with Friday's date and 07:00 with Saturday's date. The result is 8 hours. The calculator works in absolute Unix seconds internally so daylight-saving transitions do not introduce off-by-one errors.

What is "Add/Subtract Time" mode used for?

Use it when you want to know "what time or date is X hours/days from now?" For example, set the start to today at 14:00, choose Add, enter 36 hours, and the calculator returns the resulting date, time, and weekday. Handy for shift schedules, delivery ETAs, contract deadlines, or any "deadline = now + N hours" problem.

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Time Duration Calculator

Time between two dates and times in days/hours/minutes/seconds, or add/subtract any duration from a starting moment. Decimal hours included.

LIFESTYLE

Calculate the exact time between two dates and times, or add and subtract a duration from any starting moment. Returns days, hours, minutes, seconds, decimal hours, and the resulting weekday when in add/subtract mode.

Both modes work in absolute UTC seconds to avoid daylight-saving artifacts. "Between" mode parses each date+time as a JavaScript Date object, takes the difference in milliseconds, and decomposes it into days/hours/minutes/seconds via integer division. "Add/Subtract" mode adds the signed delta (days·86400 + hours·3600 + minutes·60 seconds) to the start moment and re-formats the result, including the weekday name.

Disclaimer: Times are treated as local-naive without timezone conversion. For timezone-aware calculations involving travel or international scheduling, convert both endpoints to UTC manually first.

Mode

Start Date

Start Time (HH:MM or HH:MM:SS, 24h)

End Date

End Time

Calculator information

How to use this calculator

Choose mode: Add/Subtract Duration, Time Difference Between Two Times, or Unit Conversion.
For Add/Subtract: enter start time in HH:MM:SS format, then enter the duration to add or subtract.
For Time Difference: enter start and end times. If the interval crosses midnight, check the crossing-midnight option.
For Conversion: enter a value in one unit (e.g. hours) to convert to minutes/seconds or vice versa.
Click Calculate to view the result. Output is shown in HH:MM:SS format and can be copied to clipboard.
Use for work-hour tracking, workout durations, lap times, or task scheduling.

Time Arithmetic Operations (Base 60)

T_total_seconds = hours x 3600 + minutes x 60 + seconds; reverse: hours = T div 3600; minutes = (T mod 3600) div 60; seconds = T mod 60

T = total time in seconds
div = integer division
mod = remainder of division (modulo)
1 hour = 60 minutes = 3,600 seconds; 1 day = 86,400 seconds

All time operations are performed in seconds for consistency, then converted back to HH:MM:SS format.

Worked example: Time difference from 08:30 to 17:15

Given:

Start time = 08:30:00
End time = 17:15:00
Does not cross midnight

Steps:

Convert start to seconds: 8 x 3600 + 30 x 60 + 0 = 30,600 seconds
Convert end to seconds: 17 x 3600 + 15 x 60 + 0 = 62,100 seconds
Difference = 62,100 - 30,600 = 31,500 seconds
Convert to hours: 31,500 / 3,600 = 8.75 hours
Format HH:MM:SS: 8 hours 45 minutes 0 seconds

Result: Work duration is 8 hours 45 minutes (08:45:00). Subtracting a 1-hour break gives 7 hours 45 minutes of effective work time.

Frequently asked questions

How do I add time that crosses 24 hours?

The calculator automatically handles overflow using modulo 86,400 (seconds per day). For example, 23:00 + 3 hours = 02:00 (next day). In pure duration mode, results over 24 hours display as total hours (e.g. 27:30:00) rather than 03:30. Users can choose the format that fits their need.

What's the difference between duration and clock time?

Clock time refers to a specific point (e.g. 2:30 PM = fourteen-thirty). Duration is the length of time between two points (e.g. 2 hours 30 minutes). This calculator handles both. Time minus time produces a duration; time plus duration produces a new clock time.

Does it support 12-hour format (AM/PM)?

The default is 24-hour format because it is unambiguous for calculations. 12-hour format is available in settings: 1:30 PM is automatically converted to 13:30 before calculation. Output can also be shown in 12-hour format if enabled. The US predominantly uses 12-hour (AM/PM) for everyday time.

How do I compute overtime hours?

Calculate the difference between clock-in and clock-out, then subtract unpaid break time. Under the federal Fair Labor Standards Act (FLSA), nonexempt employees earn 1.5x pay for hours over 40 in a workweek. Some states (e.g. California) also require daily overtime after 8 hours/day and double-time after 12 hours/day. Check both federal and state rules.

Can I calculate time across different time zones?

This calculator focuses on absolute time arithmetic without considering time zones. For US zones: EST is UTC-5, CST UTC-6, MST UTC-7, PST UTC-8 (standard time). For international zones, use a World Clock tool or convert to UTC first. Note that most of the US observes Daylight Saving Time, shifting clocks one hour forward in March and back in November.

Sources & references

International System of Units (SI) - Time — Bureau International des Poids et Mesures
ISO 8601 - Date and Time Representation — International Organization for Standardization
NIST Time and Frequency Division — National Institute of Standards and Technology

Last updated: May 11, 2026

Time Duration Calculator

Calculator information

📋 How to use this calculator

🧮 Time Arithmetic Operations (Base 60)

💡 Worked example: Time difference from 08:30 to 17:15

❓ Frequently asked questions

📚 Sources & references

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How to use this calculator

Time Arithmetic Operations (Base 60)

Worked example: Time difference from 08:30 to 17:15

Frequently asked questions

Sources & references