Well right now in Minecraft time our server is at Day 1289, now mind you this doesn't account for sleeping and skipping the day/night cycle. I think we started on the second of december but I could be a day or two off.
So if 1289 = D, and 1 D = 20 Minutes in the real world then: 1289 * 20 = Minutes Game has been running. (24 * 60 = Minutes in a day, 25,780 / Minutes in a day = # of days)
25,780 Minutes
or
17.9 Days of Server Run Time
Of course this time will change over time, but if you check the timestamp on this post and then do the simple math you can find out what minecraft day we're on.