With a comma it's an introductory phrase.
Without a comma it's an integral part of the sentence.
There's information here about commas after introductions.
In some sentences I run across the dates and modifiers of time separated by comma, in some sentences, however, they remain 'free'. It looks something like this:
In 1896, they moved from the island to a big city and settled there forever.
Today, there are more and more people who prefer to live in the countryside rather than in town.
In 1768 he became famous and rich and no longer needed any sort of help.
These days the country is developing very fast and noone knows what it will be in the near future.
My guess is that it depends on the length of a sentence and its rythmic structure but I'd rather learn a teacher's explanation.
Besides -- that's about what Allen 165 has just posted -- it's strongly advised that I should put a comma if it feels like making a pause after an introductory element. But it's a little bit problematic for that who learns the language, isn't it?
You don't have to put a comma there, but you can. If you put a comma there, you're changing it from a restrictive to a non-restrictive element. But it's up to you.