If you work with SQLDate(Time), you have to make sure in the first place, that you hand over your Date(Time) values in the SQLDate(Time) Format, which is YEAR-MONTH-DAY HOUR:MINUTES:SECONDS.
Xojo can help you with the translation of your TextField Date Strings, by using ParseDate:
To parse the startDate for example, you would do something like this:
theTrueBool=ParseDate(startdate.Text)
If theTrueBool Then
startdate.Text = theDate.SQLDateTime
End If
But please mind that ParseDate uses regional/international system date and time format settings on the user’s computer.