So today I discovered that the LastLogonTimestamp attribute of an account in AD (also known as LLTS) is only updated on logon if the old value is more than 14 days in the past. That means the value can only be trusted if it is more than 14 days in the past.
This, as it turns out, is very annoying if you’re trying to do some semi-half-assed auditing. Now, you can use the LastLogon attribute, which is always up to date, but that isn’t replicated between DCs so you have to query all of them.
Note to my future self: Do not rely on LLTS for anything more recent than 14 days.