Yes, duration in seconds.
I am using a helper “input_number” so I can enter minutes. In my example I use 5 (minutes).
5 * 60 = 300 but the timer is being started at 330. Trying to fix this I subtracted 30 and the timer is being started at 270.
Not sure if this a language issue, but what I meant is that there is no relation between the duration of the timer and the time at which it is started.
Think a timer on your phone: You set the timer to 5min or whatever, but that doesn’t tell you anything about the time at which the timer will start: it will start at whatever time you press the start button (actually calling the timer.start service in HA), at will end 5min later with an alarm (a timer.finished event in HA)