Issue with date and time calculations

[ksalzke]

Hello Kaitlin I found your OF template tool and I like it. It works well. And I have also a question: Is ist possible to do a calculation with dates and times in your template? I tried

                 `$DUE={{xy_day}}, {{xy_time} - 15 min`

and similar calculation ( - 00:15 or - 00.15 or - 00:15:00), but nothing worked. I only got xy_day, 00:00 in OF and not the new calculated time. The calculated time should be 15 minutes before the due date; e. g. if the due-date is 28.02.2023, 16:00 the calculated entry should be 28.02.2023, 15:45. (The date-notation is the German notation.) Thanks for your help Joe

Originally posted by @jjb53 in https://github.com/ksalzke/templates-for-omnifocus/issues/26#issuecomment-1197963032