How to make iPhone/Siri start timing?

To start a countdown timer, simply say: "Hey Siri, set a timer for [hours][minutes][seconds]"

Example: Hey Siri, set a timer for three hours, 12 minutes and 52 seconds.

The upper limit on timers is 24 hours, and you can have only one timer running on iPhone (HomePod allows multiple named count down timers now).

• You can ask Siri how much time is left on the timer by asking Hey Siri, how much time is left on the timer?
• You can stop the timer by saying Hey Siri, stop the timer.

There's no native way for Siri to start a stopwatch / count up timer, unfortunately, except through the UI.