Date Validation and accepting null values on Access Forms

Hi

I have an issue with some code behind a form, which previously worked but seem to break now.  The 3 dates on the form are defaulted and users can also manually edit it too in the field directly.  The 3 dated validate each other and the recording date has to be between the time started and time signed.

(1) Seems to be a problem with with dates manually entered through the input mask for DD/MM/YYYY HH:MM  ie 99\99\9999/99:99:0...  The criteria berow fails as macro cannot recognise a date format.

(2)  If I insert a CDate() for each field return, there will be a problem dealing with Null Values which should be accepted

Is there a better way of running these validations and also ignoring null values?

'If Me.txtTimeRecorded <> "" Then
'    If Me.txtTimeRecorded < Me.txtTimeStarted Then
'            MsgBox ("The Recorded Time can not be before the Start Time")
'            blnexit = True
'    End If
'    If Me.txtTimeRecorded > Me.txtTimeSigned Then
'            MsgBox ("The Recorded Time can not be after the Time Signed")
'            blnexit = True
'    End If
'End If