In season 2, episode 20, Asuna is disconnected from ALO by her mother because she is late for dinner. Apparently, this isn't the first time she was late, and her mother said so.
The world of SAO allows for an in-game AI-that's-classified-as-an-item/NPC (Yui)to make phone calls, or players to do their homework. Clearly there are extensions, apps, and 3rd party code available to run in ALO, kind of like with WoW here in
The Matrix reality.
Clocks and timers are usually very easy to program, and it wouldn't surprise me if something as sophisticated as a VRMMO already had a rudimentary alarm clock built in.
How/why did Asuna get blindsided by the time? Is it just a plotpoint meant for the audience to relate to being so engrossed in the game that you lose track of the time?