date Command - Formatting and Calculating Dates

2026-06-05 Reading time: About 10 min Difficulty: Beginner

What Is the date Command?

Lina: Linny-senpai, I want to put today's date in my backup file names. Do I really have to type it by hand every time?

Linny-senpai: That's exactly what the date command is for. Run it and it prints the current time, and date +%Y%m%d formats it as 20260605. You can drop that straight into a file name.

Lina: Wait, one command can build a date string for me?

Linny-senpai: Yep. And it can do more — calculate "3 days ago" or "next friday," and switch between time zones. Let's walk through the basics today.

The date command displays the current date and time, and formats it however you like. Log timestamps, backup file names, date math inside scripts — it's a command you'll reach for almost every day.

In one line

date +FORMAT builds a date string in any shape you want.

Example: date +%Y-%m-%d → 2026-06-05

What You'll Learn

Show the current time with date, and format it with date +%F and friends
Combine format specifiers like %Y %m %d into any layout
Calculate dates such as yesterday, N days later, next week with date -d
Get epoch seconds (Unix time) with date +%s and convert back with date -d @seconds
Switch time zones with TZ='UTC' date or date -u
Watch out for the syntax differences between Linux (GNU date) and macOS (BSD date)

1. Show the Current Time

Conclusion: Run date with no arguments for the current time; use date +FORMAT to change how it looks.

With no arguments, date prints the current date and time.

$ date
Fri Jun  5 15:52:59 JST 2026

The exact text (the weekday and the JST part) depends on your system's locale and time zone settings.

When you want just the date, or just the time, use the format specifiers in the next section.

2. Format the Output (%Y %m %d…)

Conclusion: Put specifiers like %Y (year), %m (month), %d (day) after date + to shape the output.

Write format specifiers after the + and you get exactly that shape.

$ date +%Y-%m-%d
2026-06-05

$ date "+%Y/%m/%d %H:%M:%S"
2026/06/05 15:52:59

If your format contains a space, wrap the whole thing in "..." (double quotes). Without quotes, the shell splits it on the space and you get an error or unexpected output.

2-1. Common Format Specifiers

Specifier	Meaning	Example
`%Y`	4-digit year	`2026`
`%m`	Month (01–12)	`06`
`%d`	Day (01–31)	`05`
`%H`	Hour (00–23)	`15`
`%M`	Minute (00–59)	`52`
`%S`	Second (00–59)	`59`
`%A`	Weekday (full)	`Friday`
`%a`	Weekday (short)	`Fri`
`%j`	Day of the year	`156`
`%s`	Epoch seconds	`1780642379`

Lina: That's a lot of specifiers. I'll never remember them all…

Linny-senpai: You don't have to. The most common combinations have shortcuts.

2-2. Handy Shortcuts

$ date +%F
2026-06-05

$ date +%T
15:52:59

%F equals %Y-%m-%d (the ISO 8601 date)
%T equals %H:%M:%S (the time)

When in doubt, remember %F (date) and %T (time) — they cover most situations.

3. Templates for File Names and Logs

Conclusion: Embed $(date +%Y%m%d) in a file name to create dated files automatically.

$(...) inserts a command's output into a string (command substitution). It's the classic trick for backups.

$ cp data.db "backup-$(date +%Y%m%d).db"

This creates a file named backup-20260605.db.

$ echo "[$(date '+%Y-%m-%d %H:%M:%S')] Process started" >> app.log

Use this when you want to add your own timestamp to a log. Example output:

[2026-06-05 15:52:59] Process started

Avoid putting %T directly in file names, since it contains a : (colon). Some environments and file systems handle that poorly. For a time stamp in a name, %H%M%S (no separators) is safer.

4. Calculate Dates (yesterday, 3 days later, N days ago)

Conclusion: Use date -d "string" to calculate dates like yesterday, 3 days ago, or next friday in plain English.

Pass the date you want to the -d (or --date) option, written in plain English.

$ date -d "yesterday" +%F
2026-06-04

$ date -d "3 days ago" +%F
2026-06-02

$ date -d "next friday" +%F
2026-06-12

You can also calculate from a given base date.

$ date -d "2025-01-01 +30 days" +%F
2025-01-31

Lina: Whenever I count "30 days from now" by hand, I always mess it up where the month rolls over…

Linny-senpai: Right, and date does that for you correctly. It handles month-ends and leap years too, so it's far safer than counting in your head.

Common expressions

tomorrow / yesterday
3 days ago / 2 weeks / 1 month
next monday / last sunday

This relative date math is a GNU date (Linux) feature. macOS uses a different syntax and these won't work there. See Section 7 for details.

5. Epoch Seconds (Unix Time)

Conclusion: date +%s prints seconds since 1970, and date -d @seconds converts them back to a date.

Epoch seconds (Unix time) is the number of seconds since 1970-01-01 00:00:00 UTC. It's a common format for storing time in programs and logs.

$ date +%s
1780642379

To convert epoch seconds back into a human-readable date, prefix the number with @.

$ date -u -d @1700000000 "+%Y-%m-%d %H:%M:%S %Z"
2023-11-14 22:13:20 UTC

When a log records only a number like 1700000000 and you can't read it, date -d @that-number converts it to a date instantly.

6. Switch Time Zones (TZ / -u)

Conclusion: date -u shows UTC, and TZ='Area/City' date shows the time in any time zone.

Add -u to display the time in UTC (Coordinated Universal Time).

$ date -u "+%Y-%m-%d %H:%M %Z"
2026-06-05 06:52 UTC

To see the time in a specific region, set the TZ environment variable right before the command.

$ TZ='America/New_York' date "+%Y-%m-%d %H:%M %Z"
2026-06-05 02:52 EDT

$ TZ='Asia/Tokyo' date "+%Y-%m-%d %H:%M %Z"
2026-06-05 15:52 JST

Lina: So writing TZ=... before the command changes the time zone just for that one run?

Linny-senpai: Exactly. TZ='Area/City' date only applies while that date runs. Your system-wide setting stays untouched, so it's safe to experiment.

Lina: That's handy for checking the time on an overseas server!

You can list the valid zone names (like Asia/Tokyo, America/New_York, Europe/London) with timedatectl list-timezones. Since TZ is an environment variable, reading Working with Environment Variables will deepen your understanding.

7. Mind the Difference with macOS (BSD date)

Conclusion: Linux ships GNU date; macOS ships BSD date, whose date math and epoch syntax differ.

The commands in this article assume Linux such as Ubuntu (GNU coreutils date). The date that ships with macOS is the BSD version, and some options differ.

What you want	Linux (GNU)	macOS (BSD)
3 days ago	`date -d "3 days ago"`	`date -v-3d`
Epoch → date	`date -d @1700000000`	`date -r 1700000000`
Parse a date string	`date -d "2025-01-01"`	`date -j -f "%Y-%m-%d" "2025-01-01"`

If a date -d ... command you found online complains with something like illegal option on macOS, you're likely on the BSD version. Run date --version; if it prints GNU coreutils, you're on the GNU version.

Mini challenge: show the date 3 days from now in YYYY/MM/DD format

Hint: calculate the date with -d, then shape it with +FORMAT.

Answer:

$ date -d "3 days" "+%Y/%m/%d"
2026/06/08

"3 days" means "3 days from now." Writing "+3 days" does the same thing.

Summary

Task	Command
Show the current time	`date`
Show the date only	`date +%F` (= `date +%Y-%m-%d`)
Show the time only	`date +%T`
Date for a file name	`date +%Y%m%d`
Calculate a date	`date -d "3 days ago" +%F`
Get epoch seconds	`date +%s`
Convert epoch to a date	`date -d @1700000000`
Show UTC	`date -u`
Use a specific time zone	`TZ='Asia/Tokyo' date`

Three things to try right now

Run date +%F to show today's date
Run date -d "tomorrow" +%F to calculate tomorrow
Run cp something.txt "backup-$(date +%Y%m%d).txt" to make a dated file