Several videos on this but overall is null on a date time function will return data with no info.  Is Not Null will do the same only data with info.  Now getting into time after 7 AM but not after 11 AM that's where the videos come in.
I recommend:
Date Time Seminar
Advanced Date & Time Calculations
Date Values
Time & Billing

I say start with Time and Billing this may answer your question and, if you are doing a much deeper dive, I say do the seminar step by step on how to get it done.  Richard does a very good job in the seminars.
Hope that helps

V/r
Juan

What are you trying to do?

Is it criteria in a Query?
Is it a Form field?
VBA code?

I use DateAdd function in a query and it works fine if there is a date. Those without a date (blank in field) turn "#Error"

I just tested DateAdd() on several DateTime fields and didn't receive any errors.

Screenshots?
- Design View (clearly showing the DateAdd() function)
- Datasheet View (showing the errors)

What is [SLdayType]? Does it have a value?

if there is no value you could use the NZ Function.
Edate: DateAdd("d", Nz([SLdayType],0), [startdate])

done. Kevin. You are just awesome