Let's say it's Sunday, and I buy a bunch of turnips, but while I'm trying to buy them, noon rolls around and now I can't buy any more turnips. If I turn the clock back an hour, will any of the negative side effects of time travel occur? Will I use my Nook Miles streak? Will my turnips spoil just because I went back an hour in time? I've already invested hundreds of thousands of Bells, so I'm not daring enough to experiment with this, but I would find it useful to know if I can at least manipulate the time of day (without crossing 5AM either way) without consequence.
Any recorded time travel will cause turnips to go bad. There seems to be no turn around to go back in time, but you can freeze time.
A possible way of freezing time. Say it's 10:00AM now and you get a good price for turnips. Your friend also want to buy it but can't be here until the afternoon. What you can do is quit the game immediately and before you open the game again, time travel to a time after 10:00AM and not before it expire.