This sounds like either the time/date is incorrect on the device OR the database is corrupted.
Double check the time (settings | date and time) and if needs be tap on the various fields to change them. (They are also set correctly when any WiFi sync is done).
If that is correct, try recreating the database. This is done by logging out from the Kobo account (settings | Accounts and then selecting Sign Out from the Kobo section). You will need to redo the setup at that time.
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