use Image::ExifTool; $err = Image::ExifTool::ShiftTime($dateTime, $shift);
1) Shift string (see below) with optional leading sign for shift direction.
2) [optional] Direction of shift (-1 or +1), or 0 or undef to use the sign from the shift string.
3) [optional] Reference to time-shift hash --- filled in by first call to ShiftTime, and used in subsequent calls to shift date/time values by the same relative amount (see ``TRICKY'' section below).
0) Shift string (and $_ contains the input date/time string).
Date-only values are shifted using the following formats:
'Y:M:D' - shift date by 'Y' years, 'M' months and 'D' days 'M:D' - shift months and days only 'D' - shift specified number of days
Time-only values are shifted using the following formats:
'h:m:s' - shift time by 'h' hours, 'm' minutes and 's' seconds 'h:m' - shift hours and minutes only 'h' - shift specified number of hours
Timezone shifts are specified in the following formats:
'+h:m' - shift timezone by 'h' hours and 'm' minutes '-h:m' - negative shift of timezone hours and minutes '+h' - shift timezone hours only '-h' - negative shift of timezone hours only
A valid shift value consists of one or two arguments, separated by a space. If only one is provided, it is assumed to be a time shift when applied to a time-only or a date/time value, or a date shift when applied to a date-only value. For example:
'1' - shift by 1 hour if applied to a time or date/time value, or by one day if applied to a date value '2:0' - shift 2 hours (time, date/time), or 2 months (date) '5:0:0' - shift 5 hours (time, date/time), or 5 years (date) '0:0:1' - shift 1 s (time, date/time), or 1 day (date)
If two arguments are given, the date shift is first, followed by the time shift:
'3:0:0 0' - shift date by 3 years '0 15:30' - shift time by 15 hours and 30 minutes '1:0:0 0:0:0+5:0' - shift date by 1 year and timezone by 5 hours
A date shift is simply ignored if applied to a time value or visa versa.
Numbers specified in shift fields may contain a decimal point:
'1.5' - 1 hour 30 minutes (time, date/time), or 1 day (date) '2.5 0' - 2 days 12 hours (date/time), 12 hours (time) or 2 days (date)
And to save typing, a zero is assumed for any missing numbers:
'1::' - shift by 1 hour (time, date/time) or 1 year (date) '26:: 0' - shift date by 26 years '+:30' - shift timezone by 30 minutes
Below are some specific examples applied to real date and/or time values ('Dir' is the applied shift direction: '+' is positive, '-' is negative):
Original Value Shift Dir Shifted Value --------------------- ------- --- --------------------- '20:30:00' '5' + '01:30:00' '2005:01:27' '5' + '2005:02:01' '2005:01:27 20:30:00' '5' + '2005:01:28 01:30:00' '11:54:00' '2.5 0' - '23:54:00' '2005:11:02' '2.5 0' - '2005:10:31' '2005:11:02 11:54:00' '2.5 0' - '2005:10:30 23:54:00' '2004:02:28 08:00:00' '1 1.3' + '2004:02:29 09:18:00' '07:00:00' '-5' + '07:00:00' '07:00:00+01:00' '-5' + '07:00:00-04:00' '07:00:00Z' '+2:30' - '07:00:00-02:30' '1970:01:01' '35::' + '2005:01:01' '2005:01:01' '400' + '2006:02:05' '10:00:00.00' '::1.33' - '09:59:58.67'
The ability to shift dates by Y years, M months, etc, conflicts with the design goal of maintaining a constant shift for all time values when applying a batch shift. This is because shifting by 1 month can be equivalent to anything from 28 to 31 days, and 1 year can be 365 or 366 days, depending on the starting date.
The inconsistency is handled by shifting the first tag found with the actual specified shift, then calculating the equivalent time difference in seconds for this shift and applying this difference to subsequent tags in a batch conversion. So if it works as designed, the behaviour should be both intuitive and mathematically correct, and the user shouldn't have to worry about details such as this (in keeping with Perl's ``do the right thing'' philosophy).