i had this last week. from looking on forums it seems to have been a bigger problem that week as lots of posts on different forums appeared round about the same day.
there were lots of solutions posted. they might work for others, they dont work for me.
1. this sticking the date forward. didnt work for me at all.
2. someone said that changing the language on your ipad would work. no great surprise that this didnt work and just left me trying to guess what was french for language, to change it back.
3. someone reported success with deleting network settings. what this did do was wipe the number of apps to update. until i opened the app store and it reappeared.
4. just scanning through all the apps in the purchased tab. all well and good but pain in the arse. it does suggest the problem though is to do with an unidentified upgrade, as in, when it says theres 3 apps to upgrade, i would only be able to find 2, and would be left with it saying '1'. which was annoying but..
5. also tried deleting apps, til i'd deleted everything and it still said '1'.
turning off and on didnt make any difference either.
eventually i wiped the whole ipad, then didnt let it restore from backup. putting in icloud settings brought back the important stuff and i then installed the apps i really wanted but not everything i had installed previously.
so far so good...