The comparisons to die games/casinos are not quite accurate, since those are competitive, while Stardew is a cooperative (and often single-player) game. The important question IMO is not "is this cheating" but rather "is everyone having fun".
In a competitive game, it's important to avoid cheating, because people will not be having fun if they lose to a cheater. And even then, I'd argue that some cheats which are agreed on by everyone can be okay. For example, when I play Monopoly, I play with unlimited houses/hotels. This is arguably "cheating", since the rulebook explicitly states that the number of houses/hotels should be limited (in order to shorten the game). But pretty much everyone I've played with prefers houses/hotels to be unlimited, because it's less frustrating that way.
In a cooperative/singleplayer game like Stardew, as long as the actual person playing is having fun, does it really matter? Sure, you can discuss what is and is not fun, but there's a really big difference between "hey, I don't think manipulating the game like that is fun" versus "you're cheating". The former is an opinion, the latter is an insult.
You'll notice, incidentally, that I didn't say "you should reset the day". I just presented an option that the original poster could use if they wanted. They're free to not do that if they don't think it'll be fun, but more information never hurts. (Personally I would find it far less fun to fail at a self-imposed challenge just because I got bad luck on a mail gift that I couldn't do anything about, but YMMV.)
@LRangerR , I specifically included this "this is arguably cheating" bit in my post in order to not get into this argument with you. Would you prefer that I not include that in the future?