Hi all, I made a base project for projects with an OLED/LCD screen and keypad but without any kind of RTC battery to keep the time. It has an edit mode where you can go through the elements of the date and time and set them directly with keypad input (invalid input is disregarded). It doesn't use increment / decrement like a standard watch because we have a keypad which is a more efficient form of input.
The input happens in the same place as where the date/time is displayed, and moves along as you input replacement segments, but will jump back to the start of the segment if it is invalid for re-entry. You can press the edit button to skip along -- if there is an invalid segment during skipping, it will go back to the original segment contents. It's not perfect because of some tiny imperfections, but it is small and it does use the small default Arduino date/time library. It's a nice trade-off.