Timers are set for a duration. Alarms are set for a time. Which makes sense btw, you can’t set an egg timer to 9:15 either, you set it for, say, 21 minutes (if it’s 8:54 right now). And you don’t set your alarm clock for “in 6 hours”, you set it for 8:00.
It’s a bit arbitrary, but this is exactly where I feel models such as Gemini or ChatGPT can actually improve things, because they can more readily leap from the keyword “timer” expecting a duration to that you actually meant “alarm” from the rest of the input, you just said timer instead.