One of the annoying features in the iOS Clock is that the minimum time for the timer is 1 minute. There are no ‘seconds’ increments that you can select from. For that most people download non-proprietary apps to get this feature. However, if you don’t want to download anything more you can still get sub 1-minute times on the Clock timer.
The work-around for this is to use Siri.
For example, bring up Siri voice command and say, “timer forty-five seconds,” and the timer will countdown from 45 seconds as commanded. Simple as that.
You can also get “in-between” times (such as 1 minute 30 seconds) as well by using the same method.