Reset
Reset
[Cueball stands in front of a doorway looking at a flip-counter sign posted on a wall.] Sign: 38 days since someone reset this sign

A common sight in workplaces is a sign reading '[XX] days since [event]' or '[XX] days without [event]', where [XX] is a two-digit number (indicated using flip cards so it may be easily changed) and [event] is some undesirable preventable occurrence. The purpose of such signs is to inspire employees by proudly displaying how long the event has been avoided or prevented. The most common version of such signs, used in industrial workplaces, displays the number of days since the last workplace accident or injury.
In the comic, the sign says "38 days since the last time someone reset this sign". The term 'reset' is the crux, because while computer-minded people tend to interpret it as 'reboot' or 'set to zero', it also means re-set (with the meaning 'set again'). See definition.
We will call: reset ~ set to zero --> meaning 1 re-set ~ set again -->  meaning 2
Because the sign uses manual flip cards, one cannot add days to the counter without re-setting the sign, which creates a paradox: either you add a day by re-setting it (which, according to the sign, means you'd have to reset the sign) or you don't (in which case the value on the sign would not be valid). The value of the sign cannot be true for more than one day. The value of the sign in the comic (38) can only be true if someone flipped the cards 38 days ago.
The sign is self-referential (which causes the paradox). Self-reference is a recurring theme in xkcd. Examples include 33: Self-reference and 688: Self-Description .
If you only use meaning 1, the sign can be seen as a challenge/invitation .
The title text refers to the signs sometimes hung over roadways in front of bridges that display the clearance of the bridge for the benefit of tall vehicles. However, this one displays only its own clearance, a number that would be unimportant if the sign itself were not there.
[Cueball stands in front of a doorway looking at a flip-counter sign posted on a wall.] Sign: 38 days since someone reset this sign
