Timers do end, and when they do, there's usually a bell. I vaguely remember my Grandmother explaining to me that after the Angelus has rung, the angels are not out there in the wide world protecting us. Or maybe it was in some kind of pious book, in the catholic lands were I was born.
The Angelus or Ave Maria may or may not have developed out of the curfew. There seems good reason to believe that a special bell, often called the Gabriel bell, was devoted to this purpose. In the Middle Ages the Angelus seems commonly to have been rung with three equal peals and this arrangement still obtains in many places.
In Rome, where the Ave Maria is sung half an hour after sunset this method obtains: three strokes and a pause, four strokes and a pause, five strokes and a pause, a final stroke.
Those strokes are for Rome, the regular angelus bell seem to be three - three - nine. I have to admit I do not remember how it sounded. I have the impression it has vanished, the bells only ringing for noon, masses, and deaths. In the city where I live now, far from the catholic lands, there is a 18:00 bell, it sounds very "end of school", but it has that end of the day feeling (and position in time).
Back to "Play worlds, not bells", I'd imagine a location centered on a rural community where, literally, bad things would happen to people outdoors after the evening bell has rung. The party reaches the village and its shrine just as the bell rings and people hurry back, the adventurers inquire and they're told "bad things are coming after the bell has rung". Later on, someone could be missing and the villagers ask the adventurers to join/lead a rescue party.
The best way to introduce the "timer" is probably to let the adventurers reach the place and having to help a local get back before the bell rings. The terror would use the NPC as a medium to try to propagate to the player characters. There could be hints of what is coming, howlings and sounds, or ominous silence. The sun sets, and then, after a while, the bell rings.