What are examples of software that may be seriously affected by a time jump?

This is a bit of open question but let me give some examples:

• databases - most of them rely a lot of precise time for storing records, indexes, etc
• security - precise time is very important for security to map action to time and gaps or time duplication is not accepted
• digital signing - usually part of signed document is the timestamp so wrong time may invalidate the signature
• scheduling software - may skip or repeat twice jobs depend of time jump direction.
• clustering software - probably any cluster will need to be in sync and any jump of one or more nodes may have unpredictable result.

All software that interacts with real-live hardware. If you have a toaster that toasts bread for 20 seconds, and its software is stupid enough to check against the wall clock, you'll either get white or burned bread if you correct the clock while waiting for your toast.

Practically all applications that control any kind of industrial device need precise timings, like, for example, "open a valve for 5.3 seconds to get the correct amount of fluid". Being off by more than a few milliseconds ruins your product.

Applications that position anything using motors will either use step motors (which are slow) or end switches to determine when to stop. But often, you don't have a switch at every important position, so you'll do some "x m/s for A milliseconds, then y m/s for B milliseconds" logic. Now imagine your NTP daemon adjusts the time by even a single millisecond while this logic is running ...

I recently got bit by a bug that dates back to 1999 and affects both the JVM and Android Runtime: https://bugs.java.com/bugdatabase/view_bug.do?bug_id=4290274

... two extra executions are fired (unexpectedly) when the system clock is set ahead one minute after the task is scheduled using scheduleAtFixedRate().

I work on a device that starts with the 1970 epoch as the current time, then receives the correct network time a little later. Occasionally a 3rd party library would initialize before the time was set, causing it to experience a 50 year time jump.

The result was scheduleAtFixedRate attempting to catch up on ~50 years worth of invocations... which was about 27 million back-to-back invocations with no delay between them.

That would cause the GC to go haywire and generally bog down the system until it was restarted

We had an issue with an on-vehicle embedded system where the clock would significant lose time (due to an electrical problem). But the wireless connections were intermittent, so the time only occasionally corrected. The upshot was that when the vehicles finally received wireless, and then an NTP update, the clock would jump forward significantly.

Various systems were checking the "last valid" time of certain things like GPS readings, etc. Suddenly all of these were "old", despite being updated only 0.5 seconds before.

Obviously a reconfiguration could fix the issue, but it was an issue.

Dovecot IMAP server is affected and (in older versions) it (deliberately) suicides if it detects the system time having jumped backwards. In v2.0, it at least tries to remedy the situation.

See https://wiki.dovecot.org/TimeMovedBackwards

It's already in a comment, but I thought I'd post it as an answer too:

Applications that should have used the steady monotonic clock but don't are also affected. For example, if software checks client keep-alives using the current time, a jump in time may kick out all clients.

I've seen regularly that software uses the wrong clock.

Plenty of examples...

• Exhibit 1 - Cloudflare leap second bug
• Exhibit 2 - F-22 Raptor date line bug
• Exhibit 3 - 787 Dreamliner engine controller time wraparound (extreme example of time going back to zero)