"Add 1 month" and "add 30 days" sound interchangeable — they're not. February has 28 days, July has 31. Mixing up duration math and calendar math is a quiet source of off-by-a-few-days bugs.
Time addition calculator, for fixed durations
This tool is for exact, fixed spans — "36 hours from now," "2 days minus 4 hours 30 minutes" — combining two durations expressed in days, hours, minutes, and seconds.
Add hours minutes seconds to a specific moment
Switch to "Apply to a date & time" to add or subtract a duration from an actual date and time — enter Sep 1, 10:00 AM and add 36 hours, and you get Sep 2, 10:00 PM exactly, no month-length ambiguity involved.
Subtract time calculator, negative results included
Subtracting a bigger duration than you started with doesn't error out — it shows a negative result (like "-3h 30m"), so you can see at a glance which duration was actually longer.
For calendar-based math — adding months or years to a date, which needs to account for different month lengths — use the Date Calculator instead.
