| This is a 2-in-1 time calculator:
duration, and time addition / subtraction.
If you fill 2 times in the first 2 rows (if you don't check am/pm,
they are in 0-23 hour format) and click "Duration" and "Calculate",
the last row will be the duration;
The 1st time "+"/"-" the number of hour/minute/second etc.
in the 2nd row (don't check am/pm in this case) equals a new time in the 3rd row.