The Apple II series (up until the IIgs) did not have a built in clock. As such, there were a few add-on devices that could be installed that would keep a battery-backed time that could be queried by software. With the introduction of ProDOS, Apple decided to add the ability for time stamping files and Apple decided to standardize on a device called the Thunderclock.
Though a few other time keeping devices came out for the machine, they were either Thunderclock compatible or needed custom software or patches to be recognized by ProDOS. In particular, one product called the No-Slot Clock was popular because it didn't take up a precious slot and also made it possible to use on the //c. However, it did require a patch to ProDOS to work.
These days, it seems that the only way to get time on your old Apple II is to purchase a reproduction of the No-Slot Clock and thus some time-dependent software seems to now be targeting that device probably because actual slot-based time cards weren't very common back in the day.
My question is whether the No-Slot Clock is compatible with the Thunderclock? I'm sure if you used ProDOS APIs to get to the time then it would be abstracted, but is it possible that people could be writing NSC software because there were extra features that could be exploited? Were they fundamentally different APIs? Does it make any sense to make software for the NSC versus the Thunderclock standard? Why did the NSC need a patch in ProDOS?