Cause
Most likely, your input format with hyphen as date separator is not registered as an input option for dates (aka “date acceptance pattern” ; see help page)
This has then caused your date/time input to be interpreted as text. Arithmetic calculations on text entries do not give a sensible number value, which is signified by that #VALUE error.
Solution
-
Change settings for date input
- Select menu item Tools - Options
- In the left pane, expand the Languages and locales branch and select General
- in the Date acceptance patterns input field, append ; D-M-Y to what you have there before.
The leading semicolon is to separate the new entry from the others that you have there
-
Your timestamp format now is useable as a source for calculation.
To make it work, either …
- Enter your timestamps again to have them accepted as proper date/time value
… or …
- Use the VALUE() function to convert text timestamps to proper date/time value
If you enter dates in this format, you may want to also apply that
DD-MM-YYYY HH:MM
formatting on your cells.