Entering a Date/Time parameter?

[321ConTact]

I am having a bad day for thought, and I can't remember how to create a
function that will return a date/time. I have a query with a date field in
"mm/dd/yyyy hh:nn:ss am/pm" format. If I enter the criteria as ">[Date1] and
< [Date2]", all's well. I want to create a function to pass the parameter
instead, but I can't remember how to do a simple thing like returning a
given date + a given time. For example, I want the [Date1] param to be
yesterday's date at 11:59:59 PM, so how would I pass this to the function. I
know this should be basically simple, so perhaps I should also ask, "WHAT
THE HECK IS WRONG WITH ME TODAY", but I'll have to ask for mercy when you
answer that one.

Thanks in advance.