That is done by adding the offset to epoch, following the scheme from the
RP2 port. RTC and `ticks_us()` are not precisely in sync, and so the
difference between `time.time_ns()/1e9` and `time.time()` will increase by
more than 9 seconds/24h. So applications should avoid using `time.time()`
and `time.time_ns()` in the same context.
Signed-off-by: robert-hh <robert@hammelrath.com>