Basically Chibi.Hoshi puts it very succinctly if you have turnips to sell (ie you have bought them)
?Time travelling forward with turnips is fine, as long as you don't pass Sunday.
?Time travelling backwards with turnips will unfortunately rot.
Separate issue: the turnip price patterns Reese offers: In past games time travelling would make the pattern go into the ever decreasing one.
Does it in this game? I'm not sure, partly because I don't TT although I freeze time to play every ingame day.
I don't think it does aka stitichmaker's post. I don't think how you change time effects it either (either via Isabelle at front screen or in system settings prior to loading the game).
Liquefy in his walkthrough faq gives a great detailed look at turnip price patterns, high spikes low spikes etc.
From his own towns (so the sample is relatively low) he only got prices over 500 4.8% of the time. But prices 200 & above happened nearly in a quarter of the weeks he played.
My towns follow these percentages. The very max price you can get is I think under 700 (I've never seen a non hacked price over).
https://www.gamefaqs.com/3ds/997811-animal-crossing-new-leaf/faqs/65246 section 32 the stalk market
If you only have one town then you are almost forced into trading (albeit if you don't you should overall still make a profit), with two towns you can keep the high priced town frozen for while to sell from the other one.
The turnip economy is to some extent 'spoiled' by players with powersave etc that can artificially put the buying price at whatever they like (I think the max is 999). It takes away some of the excitement, for me at any rate. The stalk market is a form of gambling - a good lesson as you can lose a lot as well as win.
Just a heads up to BigMikey - unless Kurt Boyer has recently improved his program for acnl turnip patterns - it doesn't work very well. His program worked very well indeed for accf, but I think Nintendo have changed their algorithm for turnip price patterns in acnl.
Can always test it out with past data, I haven't done so but I've seen several posts over the years acnl as been out saying its given incorrect advice as to when to sell.