Computing Time Differences

Author: Carl Sassenrath
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This example shows how to easily calculate the difference between two dates or times.

To compute the number of days between two dates, just subtract them. To get a positive number, put the more recent date first:

    print 28-aug-2003 - 1-jan-2000
    1335

Using this, you can do things like show how many days old you are:

    birthday: 6-jun-1982
    print ["You are" now - birthday "days old today."]
    You are 7754 days old today.

Or even how old a file is:

    print now - modified? %rebol.exe
    24

Sometimes you want to show the difference as a time (hours) not as days. In newer versions of REBOL the DIFFERENCE function will return the time difference between two date-time values:

    print difference 28-aug-2003/11:12 27-aug-2003/5:30
    29:42
    print difference now 10-aug-2003
    474:13:18
    print difference now modified? %rebol.exe
    570:05:19
    print difference now 1-jan-2000
    32082:21:41

Note that too large a time span may overflow the number of hours that can be held by the time datatype.

Also take note that the DIFFERENCE function accounts for timezone variations:

    print difference now 29-aug-2003/0:00
    18:19:51
    print difference now 29-aug-2003/0:00-7:00
    11:19:34