I don't want to use Google Apps Scripts, just formulas so that this can be display/edited on a mobile device while offline.

[![In column B, I want to put timestamps when a checkbox in Column D becomes checked. I also want the value in Column B to be null or "" when the checkbox in column D is not checked. I DON'T want these timestamps to change on all changes to the sheet, but rather ONLY when it's referent cell goes from false to true (ie unchecked to checked).

In addition, I want a formula in B1 to give a datevalue generated from the timestamp in A2, and be blank when A2 is empty. I also want this datevalue to update if A2 changes and currently it displays a blank if D2 is unchecked and checked again. But will display a date if a second checkbox/edit is made. Similarly my calculations in Column C used to determine the duration in min between timestamps will update, but if a box is unchecked and then rechecked, I have to make another/2nd edit to the sheet. Why is this happening? Is this normal? The closest thing I could find was: https://stackoverflow.com/questions/66201364/how-to-stop-freeze-pause-volatile-rand-randbetween-randarray/66201717#66201717

For the timestamp formulas I tried messing with Lambda:

=LAMBDA(x, x)(IF(B2, NOW(), )) =LAMBDA(x, x)(IFERROR(B4/0)+NOW())

but these iterate on every edit. This seems to work:

=IF(B3, IF(A3="", NOW(), A3), "")

if iterative calculation is on, but if a box in unchecked, then rechecked, it requires another edit to have any of the cells that reference it display their calculated values.

https://docs.google.com/spreadsheets/d/1KG3MbFyVzvxDG11qUmJmQsVuDwk5FI0EQg_0lx3InwY/edit?gid=0#gid=0