A true time travel is passing the 6AM mark, embarking a new day in the game and thus creating changes in your town, in shop/project developments, and in villagers perception of time. That kind of travelling is frowned upon by some.
I read somewhere that daylight savings/standard changes don't affect you if you don't play for an hour. For example, you're playing at 1:59am, and you stop to change the clock back to 1. If you don't play again until 2, it isn't TT/turnips won't spoil. Don't take my word on that without further research, especially if you have tons of turnips on the line.
Not playing for a little longer than an hour, if you set your clock back, is the best option. The game won't notice a negative difference in time between logging out and logging in, and will assume you were only gone for 5-10 minutes. Setting your clock forward won't do much unless you cross the 6AM mark.
Small changes within 24 hours won't ruin your entire game unless you do it frequently. Once you start skipping several days or even months you'll start noticing severe changes in your town and your villagers. To be 100% on the safe side, don't buy turnips if you know you're going to change time, or sell them beforehand.