Add the seconds, then carry at 60 and 24
total = Σ (h×3600 + m×60 + s) → carry 60 s = 1 m, 60 m = 1 h, 24 h = 1 day
The reliable way to add or subtract durations is to turn each into seconds, add them up, then rebuild the answer. Every 60 seconds becomes a minute, every 60 minutes an hour, every 24 hours a day. Skipping a carry is where hand arithmetic goes wrong.
The carries that trip people up
- Seconds to minutes. 40 s + 30 s is 1 min 10 s, not 70 s.
- Minutes to hours. 50 min + 20 min is 1 h 10 min, not 70 min.
- Hours to days. Once the total passes 24 hours it rolls into days, so 25 h 30 min reads as 1 d 1 h 30 min.
- Decimal is not clock. 1.5 hours means 1 h 30 min, since 0.5 × 60 = 30 min, not 1 h 50 min.
Where duration math shows up
- Timesheets. Sum daily hours and minutes into a weekly total that carries cleanly past 24 hours.
- Splits and workouts. Add up leg times and rests to get one total, seconds included.
- Cooking and steps. Chain the length of each stage to know the whole run time.
- Subtracting. A negative result means the amount you took away was longer than what you started with.
Common questions
Why can I not add times like ordinary numbers?
Because time is base 60, not base 10. Fifty minutes plus twenty minutes is not seventy minutes shown as 70; it is one hour and ten minutes, because every 60 minutes rolls over into an hour.
What happens when the total goes over 24 hours?
The hours carry into days. A total of 25 hours and 30 minutes is shown as 1 day, 1 hour and 30 minutes, since 24 hours make one day.
Does this handle time zones or daylight saving?
No. It does pure duration arithmetic on the lengths you enter. Clock shifts, time zones and daylight saving are a separate problem that needs actual calendar dates.
