Insert SQL syntax when item maybe nothing

I need to write a SQL from VBA of Excel to insert data into Access database.

e.g.
Dim strSQL as String
Dim itemDate as String                 // user will input data that is a valid date format.  such as  29/8/2007
Dim conn as new adodb.Connection

strSQL = "INSERT INTO TEST_TB (ITEM_ID, ITEM_DATE)" & _
                     " VALUES ('item001', '" & FormatDateTime(itemDate) & "' );"

conn.execute(strSQL)

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But error will appear when the user no input item date
(becuase the application is allowed no input for item date fields.

* ITEM_ID is TEXT type in database
   ITEM_DATE is DATE type in database