can you please add option so it star calculation from a full hour, so if its set on 15min it alerts at 12:15 , 12:30 ... and so on. thanks

Simple and works. THK.

good experience so far, but sadly there isn't a pause option

Interesting and good. Easy to use as a command-line extension.
- I'm wondering if a small bubble as interface could not be also useful and helpful for managing data and for a mouse-centered use, something light, with predefined buttons and time values and why not with step hours (10h, 10h30, 11h30, beginning from the current computer time).
-You should write (in extension tooltip on toolbar maybe or somewhere else) the final time (like "at 10h15" timer will end).
-A very useful feature would be a next step command : instead writing timer + min, you could say timer * or timer *. : if it is 9h52, then in first case, timer calculates itself that it has to sound at the next hour 10h00 for * and for *. it would mean that clock sounds to 10h30.
-When we click onto the icon, the time considered should not be basic 25' but the last time used preferably.
-The notification could be much more flashing to keep attention
-Change sound surely!