date and the system clock

date looks trivial and turns out to be one of the most used commands in scripts. It prints the current time, yes, but it also formats any moment however you like, parses dates you give it, and, as root, sets the clock. Underneath, Linux keeps time as a single number: seconds since 1 January 1970 UTC, the Unix epoch. Everything else is formatting.

Formatting

An argument starting with + is a format string. Each % sequence stands for a field: %Y four-digit year, %m month, %d day, %H %M %S hours, minutes, seconds, %A weekday name, %B month name, %s the epoch seconds. Anything else is copied literally, so quote the format if it contains spaces. -I is a shortcut for the ISO date YYYY-MM-DD, -R for the email-style date, and -u prints UTC instead of local time.

~% date
Sun Oct  4 18:51:18 UTC 2026
~% date +%Y-%m-%d
2026-10-04
~% date '+%A %d %B %Y'
Sunday 04 October 2026
~% date '+%Y%m%d-%H%M%S'
20261004-185118
~% date +%s
1791139878

That %Y%m%d-%H%M%S form is the workhorse of backup scripts: cp data.db data-$(date +%Y%m%d-%H%M%S).db produces names that sort chronologically with a plain ls. Avoid %d/%m/%Y in filenames (slashes are directory separators) and avoid spaces (they need quoting forever after).

Other moments: -d

-d STRING formats a date you supply instead of now. BusyBox accepts a handful of shapes: 2026-01-15, 2026-01-15 10:00, and @SECONDS for an epoch value, which is the way to turn a raw timestamp from a log or a database into something readable. GNU date also understands phrases like *yesterday* or *next friday*; BusyBox does not, so stick to the numeric forms here.

~% date -d 2026-01-15
Thu Jan 15 00:00:00 UTC 2026
~% date -d @0
Thu Jan  1 00:00:00 UTC 1970
~% date -d @1700000000 '+%Y-%m-%d'
2023-11-14

Two clocks

A PC has a hardware clock (the RTC, a battery-backed chip that ticks while the power is off) and the kernel has its own system clock, loaded from the RTC at boot and then kept by the kernel. date reads and sets the system clock; hwclock reads (-r) and sets the hardware one. As root, date -s '2031-03-01 09:00:00' jumps the system clock immediately; the RTC is untouched, so hwclock -s (*hardware to system*) copies the RTC back and undoes the experiment. hwclock -w would go the other way and save your change into the chip.

~% date -s '2031-03-01 09:00:00'
Sat Mar  1 09:00:00 UTC 2031
~% hwclock -r
Sun Oct  4 18:57:32 2026  0.000000 seconds
~% hwclock -s
~% date +%Y
2026