I also consider this an essential feature, so that collected information can be displayed in concise and readable formats. Beyond date calculations, I need robust date formatting. My standard time & date displays use one of the following formats: YYYY-MM-DD, YYMMDD or YY.MM.DD, with time optionally appended as either "-HHmm" or " HH:mm". I virtually never include day of week or the month name instead of number. Also, many times only the year or year and month are important or even relevant. Tracking books and reading is a popular theme in Tana examples, and an important thing for me in my reading history is sorting books by copyright date, which is only a year. Ideally, I could enter only a year for a date, but if I have to be more granular on entry I still don't want to see "January 1, YYYY" on output, like I've had to do in other apps.
I also consider this an essential feature, so that collected information can be displayed in concise and readable formats. Beyond date calculations, I need robust date formatting. My standard time & date displays use one of the following formats: YYYY-MM-DD, YYMMDD or YY.MM.DD, with time optionally appended as either "-HHmm" or " HH:mm". I virtually never include day of week or the month name instead of number. Also, many times only the year or year and month are important or even relevant. Tracking books and reading is a popular theme in Tana examples, and an important thing for me in my reading history is sorting books by copyright date, which is only a year. Ideally, I could enter only a year for a date, but if I have to be more granular on entry I still don't want to see "January 1, YYYY" on output, like I've had to do in other apps.
As for implementation, the Unicode forum has made code available to support this functionality for years. They have user level specifications for date/time formats, which can be found at https://unicode-org.github.io/icu/userguide/format_parse/datetime/#datetime-format-syntax. The root node for the complete description of these and other Unicode forum services is at https://unicode-org.github.io/icu/userguide/icu/.