Update form & date time field

Platform: ASP classic
DB: MSSQL

I have a datetime field on an update form that is giving me fits. When
I do not enter a value it throw an error about wrong data type. When I
put this code in

if (isnull(CStr(Request.Form("Left_Date")))) then

that processes the update without the datetime field. The else part of
the statement processes with the datetime field. The if does not work.

Any idea how this situation should be handled?

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