This is VERY serious. If a couple of years down the line your rent setting is deemed invalid during a dispute, would you be liable to pay the tenant back years of rent. Then it would be imperative to have certainty your rent setting was watertight, by taking an RTB case against yourself on its validity.As mentioned above, yes needs to be served same day, otherwise "notice is invalid". I'm not sure what the repercussions of the notice being invalid is though. If a rent review is invalid, the higher rent does not need to be paid while under dispute (link). If rent setting is invalid, does the same logic mean no rent is payable? (as rent amount not legally established). Nothing would surprise me at this point.