what have you tried so far and why didn't it work?
here is a link to the datediff function
https://docs.microsoft.com/en-us/office/vba/language/reference/user-interface-help/datediff-function
be aware that dates are stored as decimal numbers, the long integer part represent the date counting up from 31/12/1899 as 1 and today is 43362. The time element is the fractional part and is the time expressed in seconds divided by the number of seconds in a day (86400) - so 6AM is 6*60*60/86400 or more simply 6/24 = 0.25.
When using the datediff for hours you will get full hours whether they are complete or not. e.g. datediff will return 1 for a difference between 05:00 and 06:00 and 05:59 and 06:00 - if your 'one point in time' is an exact hour, it won't be a problem.
because dates are stored as numbers you can use maths instead - so for example 12am the day before yesterday and 6am today, the calc would be
43362.25-43360.5=1.75
so the time in hours would be
1.75*86400 to get the number of seconds (151200)
/60 gives number of minutes (2520)
/60 again gives number of hours (42)
the difference between 6am today and 6am tomorrow is 24 hours so add 24 to the above
edit: see there are additional posts. the easy way to get 6am today is
date()+0.25