How to Add and Subtract Time Durations
Adding time requires handling overflow: seconds overflow into minutes (60 seconds = 1 minute), and minutes overflow into hours (60 minutes = 1 hour). For example: 2:45:30 + 1:30:45 = 4:16:15 (the 75 seconds overflow to 1 minute 15 seconds, then 76 minutes overflow to 1 hour 16 minutes).
Time unit conversions: 1 hour = 60 minutes = 3,600 seconds. 1 day = 24 hours = 1,440 minutes = 86,400 seconds. 1 week = 604,800 seconds. These are the base facts for all time arithmetic.
Common uses: calculating total work hours, adding meeting durations, figuring out elapsed time during travel, and computing overtime hours. For payroll calculations, convert total hours and minutes to decimal hours (e.g., 7 hours 30 minutes = 7.5 decimal hours).
Frequently Asked Questions
How do I convert hours and minutes to decimal?+
Divide the minutes by 60 and add to the hours. Example: 3 hours 45 minutes = 3 + (45/60) = 3.75 decimal hours. This is useful for payroll and billing calculations where rates are hourly.
What is military time (24-hour format)?+
Military (24-hour) time runs from 00:00 (midnight) to 23:59. To convert PM time: add 12 to hours after noon (1:00 PM = 13:00; 11:30 PM = 23:30). Midnight is 00:00 or 24:00. 12:00 PM (noon) stays 12:00.
How do I calculate elapsed time across midnight?+
If the end time is less than the start time, add 24 hours to the end time before subtracting. Example: started at 10:30 PM, ended at 2:15 AM → 2:15 + 24:00 = 26:15 − 22:30 = 3 hours 45 minutes elapsed.
How many seconds in a day, week, month, year?+
1 day = 86,400 seconds · 1 week = 604,800 seconds · 1 month (average) ≈ 2,629,746 seconds · 1 year = 31,536,000 seconds (regular) or 31,622,400 (leap year).