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Core is an extension of Core_kernel with Unix APIs. The unmodified libraries can be found there.
In particular, Core has comprehensive implementation of times (see Time, Time_ns, Timing_wheel_float, and Schedule_v5), where some details are platform-specific (like timing wheels based on floats).
Some modules are mere extensions of those existing in Core_kernel, like Bigbuffer, Bigstring, Caml, Time, and Md5, where what's added is handlers for reading from or writing to Unix sockets and file descriptors, or support for floating-point numbers. Other modules are entirely new, like:
Command, a richly featured tool for creating command-line programs.
Daemon, for daemonizing processes.
Iobuf, which lets you use contiguous ranges of bytes for I/O purposes.
Lock_file, for managing OS-level locks.
Crc, for cyclic redundancy checks.
Linux_ext, providing a wrapper around Linux-specific system calls.
Mac_address, for managing MAC addresses.
Signal, for handling Unix signals like SIGHUP and SIGKILL.
A few modules in Core don't have any platform-specific functionality but haven't yet been ported to Core_kernel for technical reasons (like a dependency on Time, which until recently was only in Core):
This module provides support for daemonizing a process. It provides flexibility as to where the standard file descriptors (stdin, stdout and stderr) are connected after daemonization has occurred.
Module for simple closed intervals over arbitrary types. Used by calling the Make functor with a type that satisfies Comparable (for correctly ordering elements).
Latest version of Schedule, which defines a type for schedules like "every 5 min after the hour" or "every weekday at 3pm." More expressive than V4, for example by adding the Zoned_between variant, which behaves like Between but allows expressing the start and end times in different zones.
A timing wheel in which time is represented by Time.t, i.e., by a floating-point number of seconds since the epoch. This is a wrapper around Timing_wheel_ns, which is preferable both because it has better performance and because it avoids issues having to do with floating point (im)precision.
A hashtable that keeps a weak pointer to each key's data and uses a finalizer to detect when the data is no longer referenced (by any non-weak pointers).