In current version of VyOS exist utc option in firewall time:
vyos@vyos# set firewall name TESTFW1 rule 10 time Possible completions: monthdays Monthdays to match rule on startdate Date to start matching rule starttime Time of day to start matching rule stopdate Date to stop matching rule stoptime Time of day to stop matching rule utc Interpret times for startdate, stopdate, starttime and stoptime to be UTC weekdays Weekdays to match rule on
This option generate --utc flag to rule.
Option was removed from iptables in 2011 and partially replaced by --kerneltz. But, to avoid problems with dates, I think we should just remove this part and note that all times must be set in UTC.