If you haven’t already noticed, RouterOS now has name-based timezone settings, and automatic DST setting. It’s very easy, just tell it where your router is, and it will do the rest by itself:
[Demo_v2.9] system clock> set time-zone-name=Europe/Riga
[Demo_v2.9] system clock> print
time: 13:24:38
date: aug/16/2006
time-zone-name: "Europe/Riga"
gmt-offset: +03:00
dst-active: yes