converting a text field containing month and year to a date

long story short i have a data base i am building and need some advise .

the old data file is from filemaker 4 (yes fm4) and when it was built the person put the expire date for credit card in i a text field with the contents mm/yy.

now when i try to convert that data over file maker reformat's the data to mm/Dd/year. the data is useless at that . and with it still a text filed its hard search for expired cards..

i guess i need to build a script to pull off the year after the /