Don't rely on user's clock times

[miriam-rittenberg]

If you add a new class and then reload, the class sometimes disappears. This also sometimes happens if you log out and log back in. If you add two classes sometimes one of them stays and the other one disappears. This is currently in master. The branch latest-release is currently being deployed.

[miriam-rittenberg]

This was caused by my computer clock being two minutes slow, not by the new code. We should still add a warning or change something so your clock being wrong doesn't make the classes fail to save.

[georgiashay]

We should change how time works to not rely on user's clocks which may be incorrect