I just did a test using 6 textboxes and this apparently worked...
=DateDiff("d",Nz([txtExit1]),Nz([txtEntry1]))+DateDiff("d",Nz([txtExit2]),Nz([txtEntry2]))+DateDiff("d",Nz([txtExit3]),Nz([txtEntry3]))
So in the Exit and Entry textboxes with dates like this:
3/16/2020 3/18/2020
Blank Blank...