Made a quick datapack that let's you set in-game timers and thought I might as well upload it here.

You set a timer by using /function settimer:<05minutes/10minutes/15minutes/.../55minutes/60minutes>, so for example if I wanted a 50 minute timer, I'd use /function settimer:50minutes, pretty self-explanatory. Making these commands is mostly copy and paste, but for now I've only done it for 5 to 60 minutes (in 5 minute steps).

When you set a timer it is displayed to every player as a blue bossbar labelled timer (This bossbar keeps moving down as the timer ticks along, of course). When the timer ticks below 5 and 2:30 minutes you are informed in chat (and with a sound) and the colour of the bossbar changes (yellow/red). When the timer is over, the same thing applies, but instead of changing its colour the bossbar disappears until another timer is set.