This is most likely because your regional settings are not set to the proper values.
If you are using Windows XP or greater, Poker Tracker can change the settings for you at startup (you will be prompted) and then change them back when you exit the program. If it doesn't prompt you, open the Utilities > Regional Settings window and click the "Yes, always change my settings each time I run Poker Tracker" box and click the OK button. Then close and reopen Poker Tracker.
If you are using Windows ME, 98 or lower, you have to manually changed these settings BEFORE running Poker Tracker. From the Start menu, select Control Panel and then click the Regional Settings option. Language should be set to English and Country/Region should be set to United States.