Let Debatekeeper take the hassle out of timekeeping! It will sound bells automatically at the right times, or it can vibrate and/or flash your screen as quieter cues, allowing you to focus your attention on the debate.
Debatekeeper automatically switches to the appropriate speech time in styles that require it, e.g. those with reply speeches. It supports most parliamentary styles, including British Parliamentary (WUDC), World Schools (WSDC), Australs, American Parliamentary (APDA), Canadian Parliamentary, Asian Parliamentary (UADC), Australian Easters, and New Zealand university styles. More styles can be downloaded in the app from the online repository.
You can also write your own debate format (style) in XML for the app to use—see the app wiki at https://github.com/czlee/debatekeeper/wiki for details. Submissions to the online formats repository are welcome: https://github.com/czlee/debatekeeper-formats
Debatekeeper is also highly configurable with things like overtime bells and the preparation timer.
(Debatekeeper does not support the American policy, public forum or Lincoln-Douglas styles, because it doesn't support prep time between speeches distributed at teams' election, sorry.)
Advice for adjudicators: It can pay to make sure your debaters (a) know what the bell sounds like, so they don't think you just got a text message and (b) can hear the bell. It's normally loud enough in classrooms, but not lecture theatres; in those rooms you can use the vibrate/flash screen functions to remind you to clap.