Because your field is not a complete date, I doubt it will work. You'll have to either reformat the string into a format like "JUL 1, 2006" or break it down into numbers and use the Date function, something like this:
NumberVar Year;
NumberVar Month;
Year = 2000 + toNumber(right({table.Period}, 2));
Switch(
left({table.Period}, 3) = 'JAN', Month = 1,
left({table.Period}, 3) = 'FEB', Month = 2,
left({table.Period}, 3) = 'MAR', Month = 3,
...);
CDate(Year, Month, 1)
-Dell