int sd_journal_get_realtime_usec(sd_journal *j, uint64_t *usec);
int sd_journal_get_monotonic_usec(sd_journal *j, uint64_t *usec, sd_id128_t *boot_id);
sd_journal_get_monotonic_usec() gets the monotonic timestamp of the current journal entry. It takes three arguments: the journal context object, a pointer to a 64-bit unsigned integer to store the timestamp in, as well as a 128-bit ID buffer to store the boot ID of the monotonic timestamp. The timestamp is in microseconds since boot-up of the specific boot, i.e. CLOCK_MONOTONIC. Since the monotonic clock begins new with every reboot, it only defines a well-defined point in time when used together with an identifier identifying the boot. See sd_id128_get_boot(3) for more information. If the boot ID parameter is passed NULL, the function will fail if the monotonic timestamp of the current entry is not of the current system boot.
Note that these functions will not work before sd_journal_next(3) (or related call) has been called at least once, in order to position the read pointer at a valid entry.
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.