My first program was in C, input two dates in yyyy-mm-dd format, one by one,and tells the age. I used to call it Age Finder.exe Second version of it could work with dates entered in either order i.e. oldest newest or newest oldest.
Now I use javascript & html. The most used ones in recent past was my own timesheet log app. Html form submit data to Google sheet through Google script. Another page shows the monthly summary & details.
A telegram bot which logs my msgs as todo list to a Google sheet. I manually mark things done when done.
Many others at apps.bydav.in