Here's how deceitful the Revised NWT is on this Job birthday issue:
In Job 1:4, there is no context to indicate that the phrase translated "on his day" really does indicate birthdays. It is the logical conclusion that it refers to birthdays, but there's nothing concrete to point a JW to in the surrounding verses.
Until you come to Job 3:1:
Old NWT: "It was after this that Job opened his mouth and began to call down evil upon his day."
When you read that verse in context with Job 3:2, you see that Job calling down evil upon his day is him calling down evil on his birthday.
So, one could previously read these verses together and prove that the term "on his day" in Job 1:4 refers to birthdays, not just some special day.
However, here is how the Revised NWT renders Job 3:1 now:
"It was after this that Job began to speak and to curse the day of his birth."
So, now, using only the Revised NWT, you can't point to Job 3:1 to show that Job 1:4 was referring to birthdays.
Obviously, a actual student of the Bible would read multiple translations and see that Job 1:4 definitely was referring to birthdays. However, as we know, the Rank & File JW doesn't do that kind of research into their doctrine, and if they did, they're not likely to use any other translation or version of the Bible other than their own Silver Sword.