I could be mistaken, but I thought there was a package or configuration option in OpenWRT (18.06 or later) that would (a) touch a special file to update its last-modified time and (b) read that back after a reboot or cold boot to "bootstrap" the system time until NTP could take over.
To elaborate: my system doesn't have a real time clock, but can eventually get its time from other sources (NTP and some other hardware). However, in the time it takes to get to that point (~20-40s), it would be helpful to bootstrap the system date and time to something more accurate than 1st of Jan 1970. I thought OpenWRT had a mechanism to do this, by touching a file to store the current time as the file's mtime. But I can't find a package that does that, or any options in make menuconfig to enable that.
I could write it myself, but... if it's already there I'll use it. Did I imagine it? Or does such a thing exist?