I want the timesheet to only allow times in increments of 15 min. I have a
list of 96 times from 0:00 to 23:45 named ValidTimes. I set up validation on
a cell with this named list and to allow in-cell dropdown.

Selecting any value from the dropdown list validates fine.

I also want to allow times to be typed into the cell. Some values validate
OK and others do not. Below is an example:
Time Validates OK
0:15 y
0:30 y
0:45 y
1:00 n
1:15 n
1:30 y
1:45 n
2:00 n
2:15 y
2:30 n

How can I fix it so that typed in values that are in the list validate
correctly?
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Thanks for your help.