I've been tray developing for years. Here are my preferred methods:

In my U.S. darkroom I have a Zone VI Compensating Developing Timer. It has a very dim red LED readout. I place the timer on a shelf and baffle with some black board so the film processing trays cannot see the display. The timer counts up and has a footswitch. The advantage of the compensating timer is that it adjusts automatically for temperature variation (it has a temp probe). If you keep your temperature under good control (or use ambient temp and a compensation chart), any timer with this sort of display will work fine; the footswitch is handy though.

In my darkroom in Vienna (actually my interior bathroom converted for film processing when needed) I use a combination of cheap kitchen timer and a metronome. The metronome is set to 60 bps (one beep every second) and I could just use it alone if I wanted to keep track of counting the seconds during the entire processing time (I have done this and do for stop and the first part of the fix). However, setting the kitchen timer to the developing time (minus drain time) allows me to just use the metronome to time my agitation. I start the timer, immerse the sheets, agitate with the metronome and, when the timer sounds, get sheet one on the bottom of the stack, lift and drain the negs and transfer them to the stop.

In the stop I count 60 seconds on the metronome after shuffling through the stack once. I then transfer to the fix and count seconds till I'm halfway through the fixing time. I then switch on the white light so I can see the clock and continue fixing using that.

I kind of like the idea of using different pieces of music for timing the process. Heck if I could find a bunch of pieces that had 60 or 120 bps tempos and matched my N, N+ and N- times I might adopt that method; then I could use the tempo to guide my agitation much as I use the metronome now. And, anticipating the end of the piece would help with getting everything ready to get out of the tray on time.

I do need something that either shows or sounds seconds though, so I can keep track of my agitation.

I've had bad experiences with my cell phone in the darkroom, so I keep it in a dark pocket on my black leather darkroom apron now...