Do these all have to be used on the same day?
Absolutely not. I think the date can be up to a year from purchase - not absolutely sure but you can definitely use them within a week or a fortnight.
They're good indefinitely...according to their website.
I've used them well over a year old, maybe more.
The biggie you have to be careful about - Don't mix up the used tickets with the unused tickets! It's usually VERY difficult to read the printing on the back on your validated (used) tickets... Come up with a plan, for instance - unused tickets ALWAYS in left pocket, used tickets ALWAYS in right pocket - to keep them apart. At the end of the day, throw away or physically separate those used tickets from your daybag!!!
Eileen Good point. I agree that the one failing of the T-carnet is keeping the old from the new. I now I have had it backwards at least once. I tried using a rubber band but it broke.