Convert text date

I am creating database from Excel file. The dates in this Excel file were recorded in nonstandard format such as:

January 6-7-2013

The "6-7" represents two days: January 6 & January 7.

I would like for Filemaker to ignore the second day ("7") and create the date as:

1/6/2013

How would I do this?

(I've already imported the entries into Filemaker from Excel and have 3212 records...far too many to change manually by hand. Can I just convert them all inside Filemaker or should I just reimport them using a calculation field of some sort to convert the nonstandard date format to a standard date format?)