Because in VB trere are no DateTime data type, you may speak for variable of Date data type (which can contain also time) or for Variant variable. Anyway, there IsDate() function, which you can use to check if entered value (including time) is valid Date:

Dim SampleDateTimeValue As Date
Dim IsValidDateTime As Boolean

  SampleDateTimeValue = DateSerial(2002, 12, 2) + TimeSerial(11, 34, 45)
  IsValidDateTime = IsDate(SampleDateTimeValue)  'Returns True
  SampleDateTime = CDate(txtDateTime.Text)
  If Not IsDate(SampleDateTimeValue) Then
    MsgBox "Please enter valid date and time!", vbExclamation + vbOKOnly, "Error"
  EndIf