Problem saving datetime to file and reading it back for a calculation

[Steve]

I would like to use the line:
HoursDiff = int((d2-d1).total_seconds()/3600)
to determine the difference in hours between two timedate entries.

The variable d2 is from datetime.now()
and d1 is read from a text file.

I can save d2 to the file only if I convert it to string and, at a later
date, it gets read back in as d1 as string.   The variable d1 as string will
not work in the HoursDiff statement.

To me, it looks like a problem in formatting.
How do I fix this?

Steve