I searched the API but didn't find it there. At one point I ran across something somewhere that said when Lua was implemented, os.time() was
specifically overridden to provide game-time instead of real-time. I.e., this is intentional, not an oversight.
That said, os.day() returns a number and os.time() returns a number. A Minecraft day is 20 real-time minutes. I don't have my code in front of me right now, but I've written a simple function that returns elapsed real-time if you provide the start day and start time. That's useful for determining how many minutes (or hours) a mining operation took.
Extending further, you can write a time server that keeps the master time (I guess it would take human input) and broadcasts this to all other turtles, which would then sync their time to the master. Or aren't there real-world time RESTful servers that return time in XML or JSON or something? Your program can ping them to sync their internal time with real-time at the beginning and then do a conversion (0.0139 real-days to 1 Minecraft-day).
A quick search found this RESTful API:
http://api.geonames....demo&style=full which returns, among other interesting things, <time>2014-02-04 13:28</time>.