Place a control on your form that contains your DoCmd.OpenReport method and
try something like:
Dim strWhere as String
strWhere = "1 = 1 "
If Not IsNull(Me.txtStartDate) Then
strWhere = strWhere & " And [DateField]>=#" & Me.txtStartDate & "#"
End If
DoCmd.OpenReport "rptYourReport", acPreview, , strWhere