Its is fine if you want to time-travel. To make a blanket statement that its okay is absolutely wrong to do and mis-guided. There are many reasons why one shouldn't and the most important is if there are others who use the same computer.

There are programs on my computer like calendars that are affected when someone plays with the system clock.

I'll say this, if my kids (who use my computer) moved my system clock; I would be quite angry. So angry that they would be banned from the computer.