You're fine so far. A little varience of only a few minutes is probably not going to affect your plants. However, if you're serious about growing, it's great to get a timer because they are mechanical and will do work for you, and will be more precise than even the most careful person. However, if you're worried about your timer's reliability, then you can take a few steps to ensure that it's always working. First, make sure to get a quality timer and plug it into an outlet that you trust. I recommend getting a digital timer because they are harder to mess up when you're programming then, since they display the time right on the screen. Second, you can still check on your plants every day to make sure everything's working okay and the lights are going on and off as planned. It's a good idea to check on your plants daily anyway, so you can plan to check on then right after or right before the lights change.
If you talk to your friends who had problems with their timer, it was probably because of one of these things: their old crappy timer broke, they didn't program the timer properly, or they used a bad outlet (or an outlet that can be turned off with a switch). In any case, if they had been checking on their plants dailey, they wouldn't have had problems with their plants. It's almost always okay if something goes wrong with the lights for one or two days. Your plants won't even react. You end up getting problems when the plants aren't getting a consistent light schedule over time.
Happy Growing!
