Check presense of date/Time field? | IF in DateTime field gives syntax error

Hi there QB-community,

I'm trying to build something around timekeeping.
In my design I have 2 date/time fields to get a start DateTime value from the user.
One field is created as a timestamp the other field is created by manual entry.
[StartTime_TimeStamp]
[StartTime_Manual]

Via a third formula date/time field [StartTimeFinal]  I like to use to most preferred start time in further presentation and reporting purposes.

So basically i look for an IF function to set the [StartTimeFinal]
in human logic something like:

IF [StartTime_Timestamp] exists than [StartTimeFinal] = [StartTime_TimeStamp] else
IF [StartTime_Manual] exists than [StartTimeFinal] = [StartTime_Manual]

But using the IF function in a date/time field gives a syntax error "expecting DateTime"

Who can give me some suggestions ?

Thanks Jan-Willem