If you're getting the time the report was run, you can use the global variable Execution Time.

=Globals!ExecutionTime

You could get 12 hours before that using this:

=DateAdd(DateInterval.Hour,-12,Globals!ExecutionTime)

If you create a placeholder for each of the values, you can format them as Long Date (date and time).

If you're getting the date/time value from a parameter, you'd just substitute Parameters!ParameterName for the Globals!ExecutionTime