While it's great that people get their games working by reinstalling, removing various mods etc, I feel this thread may give new visitors the idea that the bug is fixable by the player. As someone who has had this problem since updating and have been reading this thread for a week now, these are, as far as I can tell, the facts:
1. The bug is unpredictable. It is entirely possible to play for a long while, save multiple times successfully, with the bug still being present. That has happened to me and many others. But eventually, the game still resets, unless you just happen to be very lucky. Thus, the fact that your game currently works does not mean that whatever you did fixed it. I wish it did.
2. Mods can trigger the symptoms of the bug more frequently but it is, unless players are lying, present in both console and unmodded games.
3. The fix for the Mod That Must Not Be Named is an attempt at making that specific mod work around the data points that appear compromised, not an attempt to fix the bug in itself. Only the devs can fix the bug. (Or rather, the devs can hopefully fix it. This is an old game that was, apparently, originally developed to be something very different and then hastily modified to resemble older TS games.)
4. The bug seems to be primarily related to outfit data but causes cascading issues.
5. The bug is in the base game but may be made worse by HSY. I'd guess because of Trendi.
6. The players can't fix this bug. At most, we can try to avoid triggering it.
7. The bug most likely affects many more people than visit this thread. Just on Reddit, I've seen multiple people either describe the save issues or other problems that seem strongly related to this. It's entirely possible that many players who are not specifically trying advance through, for example, high school or just play more 'relaxed' games where they mostly stay on one lot, might not even notice that the days reset but only that weird things are happening to relationships and skills.
8. This bug is most likely connected to a bunch of other symptoms like settings not saving, unclaimed babies and inexplicable routing/teleporting by sims.
In short, this appears to be a very serious issue that the devs must fix. And, considering its symptoms suggest serious problems with data handling, it may take a long time, though I certainly hope it doesn't.
