I like this as a rough idea of the time. It is likely to be used in the small hours by the vast majority, so perhaps avoid 24hr clock, but the LED colour could be used to interpret this. Seriously, though. If you don't know already if it's am or pm right now, you need to get out more! The hours could be represented by long flashes, followed by either one, two, three or four short flashes to signify which quarter of the hour you're in e.g. flaaash, flaaash, flash, flash would mean between quarter past and half past two.