According to the IRCC page, there is no specific date requirement on the Proof of Funds letter.
Here is what they say (relevant part below):
Document requirements
- The applicant should provide an official letter(s) from one or more financial institutions that lists all current bank and investment accounts as well as outstanding debts, such as credit card debts and loans.
- The letter(s) must be printed on the letterhead of the financial institution and must include the applicant’s name, the contact information of the financial institution (address, telephone number and email address), the account numbers, the date each account was opened and the current balance of each account as well as the average balance for the past six months.
Obviously, there is a limit here because no one would accept a letter dated 6 months ago as proof of funds. I think having it dated as recently as possible is best. Mine was dated a few days before my ITA, but I applied very quickly after getting ITA.