Thanks Paul,
To avoid rebuilding the entire (very long) query in code I put this in the appropriate column in the criteria row in the Query module:
Code:
>=DateValue([Forms]![frm_DowntimeReasons]![txt_FromDate])+TimeValue([Forms]![frm_DowntimeReasons]![txt_FromTime]) And <=DateValue([Forms]![frm_DowntimeReasons]![txt_TillDate])+TimeValue([Forms]![frm_DowntimeReasons]![txt_TillTime])
To my surprise it worked.
Here's a bonus question. Is there any kind "time picker" similar to the Date Picker property in the text box?