How to get days in decimal from two timestamps

The date/time stamps in columns A and B are text strings, not real dates. Convert these to real dates, and your subtraction formula should work just fine. Some strategies for converting numbers (and dates) stored as text to numbers: https://support.microsoft.com/en-us/...rs=en-us&ad=us

I used the Text to columns feature, removed the time zone indicator, and Excel correctly interpreted the date/time stamp, and then the formula in column C works just fine.
1) Select A2 -> Text to columns -> Fixed width.
2) Add/Delete breaks so that the date/time stamp is in one column and the time zone is in its own column.
3) Tell Excel to skip importing the time zone column -> Finish.

Repeat for B2. If you need to retain the time zone information, have the text wizard output the columns into helper columns somewhere off to the right.

Will something like that work for you?