Calculate difference between two Times

Question

Given two Time variables (not DateTimes), how can I calculate the difference (in minutes, seconds, or milliseconds, doesn't matter) between the two?

Time startTime = user.Appointment_Start_Time__c;
Time endTime = user.Appointment_End_Time__c;
Long timeAvailable = (endTime.minute() - startTime.minute());
System.debug(timeAvailable);

This returns a -9 with the following variables:

startTime | 35100000
endTime | 48600000

and these user assignments:

If I use millisecond() it returns a 0. What's going on?

Answer 1

The hour(), minute(), second(), and millisecond() methods return the value as you'd expect if you were telling someone what time it is. For example, if I say it is ten minutes past six (assume early morning), hour returns 6, minute returns 10, etc. hour returns a value from 0 to 23, minute from 0 to 59, second from 0 to 59, and millisecond from 0 to 999.

To get a difference in time, subtract the milliseconds via getTime:

Long timeAvailable = endTime.getTime() - startTime.getTime();

The getTime method returns the number of milliseconds since Jan 1st, 1970, midnight GMT. This math gives you a difference in milliseconds. From there, to get the minutes, divide by 60,000:

timeAvailable /= 60000;

Note that even though milliseconds are an attribute of datetime, the database does not currently store or return milliseconds, so millisecond() called on a value from the database will always be zero.

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Full Example:

DateTime startTime = DateTime.newInstance(Date.today(), user.Appointment_Start_Time__c);
DateTime endTime = datetime.newinstance(Date.today(), user.Appointment_End_Time__c);
Long timeAvailable = endTime.getTime() - startTime.getTime();
timeAvailable /= 60000;
System.debug(timeAvailable);

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Without converting to DateTime values, you need to calculate the hours and minutes, and subtract the difference;

Time startTime = user.Appointment_Start_Time__c;
Time endTime = user.Appointment_End_Time__c;
Long timeAvailable = (endTime.hour() * 60 + endTime.minute()) -
                     (startTime.hour() * 60 + startTime.minute());