Add or subtract two time durations, or add/subtract time from a starting date and time.
Every value is first converted to seconds, added or subtracted, then converted back into a whole day/hour/minute/second breakdown — the same way you'd carry over units by hand (60 seconds roll into a minute, 24 hours roll into a day).
Subtracting a larger duration from a smaller one produces a negative total, shown with a minus sign in front of the day/hour/minute/second breakdown.
Yes — any blank field is treated as zero.
Yes — because the "add/subtract from a date" tool works in your browser's local time zone, results correctly skip or repeat an hour across a daylight saving transition, just as a clock on the wall would.
Pick another tool to jump straight to it.