Посмотрите варианты вывода через ключ -o.
-o --output=STRING Change journal output mode (short, short-precise,
short-iso, short-iso-precise, short-full,
short-monotonic, short-unix, verbose, export,
json, json-pretty, json-sse, cat)
например
journalctl --output=short-iso
выдаёт такой формат
2018-01-28T18:58:20+0300 ubuntu kernel: BIOS-e820: [mem 0x00000000bfd15000-0x00000000bfd5ffff] ACPI NVS
2018-01-28T18:58:20+0300 ubuntu kernel: BIOS-e820: [mem 0x00000000bfd60000-0x00000000bfd68fff] ACPI data
2018-01-28T18:58:20+0300 ubuntu kernel: BIOS-e820: [mem 0x00000000bfd69000-0x00000000bfd6bfff] ACPI NVS