Here's something to get you started. You will have to look into converting it to fit your form.
If you're on an older release that haven't got the ADD-INTERVAL function you can use the DATE function and add +1 to the month instead (just remember that if the month is December (12) you're going to have to increase the year instead and set the month to 1).
DEFINE VARIABLE cFromDate AS CHARACTER NO-UNDO.
DEFINE VARIABLE dtFromDate AS DATE NO-UNDO LABEL "From".
DEFINE VARIABLE dtTomDate AS DATE NO-UNDO LABEL "Tom".
DEFINE VARIABLE iMonth AS INTEGER NO-UNDO.
DEFINE VARIABLE iYear AS INTEGER NO-UNDO.
UPDATE cFromDate FORMAT "99/9999" NO-ERROR.
IF LENGTH(cFromDate) <> 6 THEN DO:
MESSAGE "Invalid format" VIEW-AS ALERT-BOX ERROR.
RETURN ERROR.
END.
ASSIGN
iMonth = INTEGER(SUBSTRING(cFromDate,1,2))
iYear = INTEGER(SUBSTRING(cFromDate,3,4)) NO-ERROR.
IF iMonth < 1 OR iMonth > 12 THEN DO:
MESSAGE "Invalid month" VIEW-AS ALERT-BOX ERROR.
RETURN ERROR.
END.
IF iYear < 1990 OR iYear > 2020 THEN DO:
MESSAGE "Invalid year" VIEW-AS ALERT-BOX ERROR.
RETURN ERROR.
END.
ASSIGN
dtFromDate = DATE(iMonth, 1, iYear)
dtTomDate = ADD-INTERVAL(dtFromDate, 1, "month") - 1.
DISPLAY
dtFromDate FORMAT "9999-99-99"
dtTomDate FORMAT "9999-99-99".