This quote is commonly attributed to Albert Einstein, especially in:
"The only reason for time is so that everything doesn't happen at once."
However, there are many cases of quotes misattributed to Einstein. There are many sources that use this quote, and it's quite popular in image form (as shown by a simple Google Image search). There is no mention of it on Albert Einstein's Wikiquote page, which is otherwise pretty thorough, from what I can see.
What evidence is there that Einstein really said this?