You can use hwclock which should be built into every embedded linux distro.
$hwclock is - as the command name suggest - a hardware clock, independent of the OS being used. These days it uses a low-power oscillator built into the motherboard, that tracks the passage of time even when the machine is powered down (uses mb battery that will last a long time)
If hwclock is not available, there are other alternatives which I will gladly go over, but hwclock should be your goto command if available.