Please read this: You are NOT at fault, and might be dealing with a scamming landlord.
There is no set expiration date on checks, and even a line in the memo (i.e., void after ninety days) is not necessarily honored by a bank.
Six months is the unofficial limit, beyond which a bank might not honor a check, but request a new one be issued to replace it.
Hypothetical situation: a landlord takes a check to the bank 8 months after the date. The bank might call the writer of the check to ask if it is legit. The writer says no, the bank does not honor the check, the landlord claims the tenant is in arrears and files for eviction.
The result in court might be the landlord used deceptive practices to trick the tenant- banks foreclosing on rental properties often use this tactic to trap tenants and then try to evict them for non-payment- so do crooked landlords. They won't ask for rent for six months, then request it, in the hopes that like many people, you would not have the foresight to hold on to it.
A judge might rip them a new one. They don't like people who play games.
As a tenant, if the situation is such where a landlord or owner does not cash checks (or requests them), place the rent money in a separate account- a joint account with other tenants, if that is the case. This is something you can show to a judge if a landlord is playing games. This way you can demonstrate an effort to make certain you have made the effort to cover the rent. And if a landlord or owner is playing games, they will get busted. Trust me, I had this happen in court with a sleazy foreclosure law firm. I humiliated the liars in court.
PS: I am probably overreaching on this, but they may simply (at most) be trying to get you for late fees on this. You definitely need to keep cash available for any checks you've written- I'd say minimum one year- the company might just be very inefficient. With friends/housemates, if a check is more than two weeks old I ask them before depositing it. It's a common financial mistake that people make. But I always prepare for worse-case scenarios, i.e., the possibility the landlord may be playing games.