You get 90 days when you load the cartridge to use the 50 tests. The Validity Date is calculated from the date you insert the cartridge into the machine. The date is stored on the cartridge, so if you remove it and re-insert it, the meter will remember when you first loaded it. One of the options on the menu shows the number of days left to the end of the 90 day validity period. It does give you a warning as you approach the Validity Date. You get one days grace period at the end of the validity period.
You can't use the cartridge if it is after the Expiry Date or the after the 90 day Validity Date. So don't bother buying cassettes of strips which will expire before you need to use them as they wont work.