Combine date string for DateDiff

[ReinerKnudsen]

I have a date picker for my date (date_Von) and a text field for my time (txt_zeitVon). 

I have two sets of these (date_Von and date_Bis, txt_zeitVon and txt_zeitBis) and want to compare them with DateDiff.

My problem is that building a valid expression for DateDiff fails.

I try 

DateTimeValue( 
        date_Von.SelectedDate & " " & txt_zeitVon.Text)

as well as

DateTimeValue( 
        date_Bis.SelectedDate & " " & TimeValue(txt_zeitBis.Text)) 

What is really funny is that DateDiff works for both of them, but PowerApps always gives me an error saying:

The arguments to the DateTimeValue function do not represent a valid date or time value.

Any idea why PowerApps won't like my expressions?
Thanks for your help.

[RandyHayes · Accepted solution]

@ReinerKnudsen 

Convert your date to text in your formula...

Please consider changing your Formula to the following:

DateTimeValue( 
        Text(date_Von.SelectedDate, ShortDate) & " " & txt_zeitVon.Text
)

I hope this is helpful for you.