If the DNSSEC validating resolver is a server then it is usually not an issue. Most server hardware has battery backed real time clocks. In the odd case that you are bootstrapping a server you would have to set the time manually or make setting the time part of bootstrap process.

For embedded systems that don't have a battery backed real time clock and want to do local DNSSEC validation this is indeed an issue.

There are plenty of hacks to make it work, but no real standard.