Open "A Filter". The maximum number of characters that can be typed into the "Timeshift" box are 5, including an eventual trailing '-'. This limits timeshift functionality to 10 seconds if negative and 100 seconds if positive.
If you really think this is gentoo specific, please re-open and tell us why. Until then, I'll mark this as 'upstream' and ask you to tell the developers of the program about this problem. Thanks!