Comment 5 for bug 1878422

        /* Let's try to make sure that the clock is always
         * monotonically increasing, by saving the clock whenever we
         * have a new NTP time, or when we shut down, and restoring it
         * when we start again. This is particularly helpful on
         * systems lacking a battery backed RTC. We also will adjust
         * the time to at least the build time of systemd. */

So i think the "epoch" of when systemd is built + the stored /var/lib/systemd/timesync/clock achieves the equivalent functionality of increasing time between boots. And ensuring the very first boot is "recentish".