6ヶ月以内という規定はあるようですが、義務ではないので、銀行がそれでOKなら受け付けてくれることもあるようです。一応、その銀行に問い合わせてみることをお勧めします。
Canadian Payment Association のFAQページより抜粋
http://www.cdnpay.ca/faqs/cheques.asp#7
When is a cheque considered stale-dated, and is it no longer valid after that point?
Under CPA Rule A4, Section 21, a cheque is considered stale-dated after six months, unless it has been certified. Although it may be returned through the clearing for that reason, there is no obligation to do so, and the payor’s financial institution may still accept it as a valid payment item. An institution accepting a cheque that is dated more than six months earlier may choose to contact the payor’s financial institution to confirm whether the cheque will still be accepted.