Those are the options for “epoch”, i.e. the date which counts as the “day zero” reference point. Changing this will have zero effect on formatting, but will change all calendar dates and mess up your data. Have the first item selected, except for very special cases (old spreadsheet files from Apple platform or from early versions of StarOffice, mostly)
Date/time values are simply numbers with a date format applied. When you type something which is interpreted as a date, a corresponding date format is activated for the entry “behind the scenes” (not explicitly applied as cell format) unless there is specific number/date format already applied on the cell.
This is part of the “default format” behavior, translating the input date into a “count of days from epoch” and formats it as closely as possible to appear as you typed it in. You get what you ask for, as it were.
Interpretation of input
This is governed by language choice, but after selecting language you can modify the set of “input mask” codes.
- See menu item Tools - Options
- In left pane select Language settings - Language
- Insert the desired
Y M D;
Y-M-D pattern(s) into the
date acceptance patterns field. You can have several patterns separated by semicolon.
Note that this field will reset to the defaults for the chosen language/locale every time you select a different language.