The current logic looks like it was effectively rounding the elapsed time to the nearest hour (with a 30-minute bias). That means a file only 30 minutes old could be considered "1 hour old," which isn’t what a user would normally expect if they configured rotation every hour. A user would reasonably expect “1h” to mean exactly 60 minutes, with perhaps a small tolerance for cron jitter or a slow rotation run.

I like the change in general, but if I was you I'd probably use getopt_long() and call the option --signal (consider this as an optional suggestion; I don't have strong opinion with this choice), because -I was somewhat counterintuitive to me as a flag to specify signal and both -s and -S were already occupied.

For those examining the history of newsyslog(8) and the removal of the -c command-line option, I feel it's important to understand the underlying design philosophy that motivated this and related changes. The goal was to establish a more robust, flexible, and administrator-friendly configuration model.

I think it makes sense to just leave compression to on (use the default, which is lz4 for the current ZFS version) instead of explicitly saying lz4 here because that would be more future proof. The change looks otherwise fine to me.

Looks good to me overall, thanks for working on this! Please fix the free() operation in inline comment.

I don't have real hardware at hand but the change looks fine in principle.