ntpd

If ntpd finds that the clock is ahead, it will slow the ticks, allowing it to gradually drift back to the correct time.

That might cause some extra wear on systems running on flash storage and it made me install ntpd instead.

You're thinking of nptdate, a utility provided with ntpd, but which is only ever supposed to run at system initialization time to get the time close enough for ntpd to take over.

The only truly lasting pieces of code are the really basic building blocks, stuff like ntpd.

When you install ntpd and similar you'll find it almost always comes configured with a vendor pool for the distribution e.g.

User intervention or ntpd can cause the system clock to go back in time.

This will break if the system clock changes, either from user intervention or ntpd.

What if I disable ntpd a minute before leap second is injected and restart it a minute later?

Overall far fewer mystery sync issues than the reference ntpd though, nice software.

The aforementioned thread[0] on Serverfault mentioned timesmearing with `ntpd -x`, as per [1].

Local edits, updates not applied, literally ntpd/ntpdate not updating the time.

I believe it's not reliable as it can be changed by other processes running alongside your application (like ntpd for example).

It's not as precise the ISC ntpd, but normally "good enough".