I haven't found it, but I can tell you what I've tried, short of just lock picking 10,000 possible combinations.
I've tried stuff like 3:40-3:50, which would roughly match the clock, even if the hand positions don't make sense.
I've tried numbers alluding to other things:
1920 - the roaring 20s.
1929 - the stock market crash.
1984 - like the novel, kind of a stretch, since this world is more business than surveillance.
1849 - the year the bowler hat was invented, due to the painting on the wall.
I've tried 9:00 and 5:00, because 9-5 working hours, right?
I've tried using rat's punch code and year of birth.
I've tried using the issue date of rat's id card.
I've tried using the real time on my desktop.
Looking at the number of protruding branches on the trees in the room, from left to right, it was a little hard to tell, but it sort of looked like 2-4-3, so I tried 2:43.
I've tried 11:59 and 23:59, which would be close to midnight, depending on if we're using military time or standard time.
I've tried the amount of money I have on me.
I haven't gleamed anything useful from what I could inspect in the room. I've tried inspecting things after changing the time, but I haven't noticed any changes.
I've tried 4:00, since 4 is written on the calendar item.