24 hour time calculator
After entering a time in 24hour format, this calculator lets you add and subtract a period to give a new time,
also in 24 hour format. The 24 hour time format is also known as "military time".
For example, you could enter the time 15:00:00 (3 pm),
then add 70 seconds to it which would give 15:01:10 ( 3:01 and 10 seconds pm).
Instructions
The calculator adds or subtracts input durations from the time in the top window. You first enter a time in 24 hour format
For example 6:12:0 which is read as 6 hours 12 minutes and zero seconds. (6:12 am).
After that you enter durations and add/subtract them from the time in the top window.
 Enter a time in three parts: the hours in left field, minutes in the middle, and seconds in the right field.
When entering a time, the calculator checks that the hours are in the range 023, minutes and seconds are in the range 059.
 Enter the time by pressing "Enter a time". The time is placed in the top window, replacing what was there.
 Enter a duration in the the three input boxes. The inputs can be any positive number.
For example you could enter 100 in the minutes box and press "+".
This would add 100 minutes to the time and display the new time.
The numbers can have decimal places. For example 3.5 minutes will be taken as 3 min 30 sec.
How it works
When a time is entered, it is internally converted to the number of seconds past midnight.
When a duration is entered (For example: 2 hrs 8 mins) the duration is converted to seconds.
The addition / subtraction is performed on the seconds and then the result formatted as a new 24  hour time.
If the operation results in a time in another day, the number of days offset from the original time input is also shown.
For example if you enter 3:00:00 and then subtract 4 hours, it will show the result as
"23:00:00 Previous day"
meaning 11pm the day before the original time you entered.
A note about midnight
The calculator shows midnight as 00:00:00, and assumes that to be the beginning of a day. It will never display 24:00:00.
So for example
 00:00:00 + 8 hours = 8 o'clock in the morning on the same day
 10:00:00 – 10 hours is 00:00:00 on the same day
 23:59:59 + 1 second = 00:00:00 on the next day
12hour readout
All time outputs are also displayed in 12hour format below the main display window.
It shows midnight as 12AM and is assumed to be the beginning of the day.
To avoid confusion, midday is shown as "12:00:00 Noon" It never displays "12 PM". So for example
 10:00:00 + 2 hours = 12 Noon
 23:59:00 + 1 minute = 12:00:00 AM next day
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