Date/time field bahaving strange on save

  • I have a form with a date/time field regDate. If I select the date in the field and save the document it is OK, But if later I open the same document and save it without editing the field regDate I receive:
    Validation failed
    regDate must be a date/time (submitted value was: 2011-03-18)

    If I select the date again (the same date) everything works.

    Any suggestions how to fix this?