date

What it does

date prints (or, with root privileges, sets) the current system date and time. Its real power comes from the +FORMAT syntax, which lets you print the date/time in any custom layout using format specifiers like %Y (year), %m (month), %d (day), %H (hour), %M (minute), and %S (second) — extremely common in scripts, logs, and generated filenames.

Beginner examples

• date — print the current date and time in the default locale format
• date +%Y — print just the current year
• date +"%Y-%m-%d" — print the date as 2026-07-05
• date -u — print the current time in UTC

date +"%Y-%m-%d %H:%M:%S"

Advanced examples

• Generate a timestamp safe for filenames: date +"%Y%m%d_%H%M%S"
• Compute a relative date: date -d "+7 days" +%F or date -d "last monday" +%F
• Print ISO 8601 date only: date -I
• Print RFC 5322 format (used in email Date: headers): date -R
• Convert a Unix epoch timestamp to a readable date: date -d @1751500800
• Get the current epoch timestamp for scripting: date +%s

date -d "+7 days" +%F

Common mistakes

• Forgetting quotes around format strings containing spaces, e.g. writing date +%Y-%m-%d %H:%M:%S instead of date +"%Y-%m-%d %H:%M:%S" — the shell will treat the unquoted part after a space as a separate argument and date will error.
• Using GNU-only -d/-s relative-date syntax on macOS and getting a syntax error, since BSD date uses -v for adjustments instead.
• Assuming date automatically accounts for timezone conversions without -u or TZ=, leading to confusion when comparing timestamps across servers in different zones.
• Manually setting the clock with date -s on a system that also runs ntpd/chronyd — the time service will likely override the manual change shortly after.

Tips

• Use date +%s to get Unix epoch seconds, handy for timestamping log entries or comparing durations in scripts.
• date -d "@<epoch>" converts an epoch timestamp back into a human-readable date.
• For portable scripts that might run on both Linux and macOS, avoid GNU-specific -d relative syntax, or detect the platform and branch logic accordingly.

Best practices

• Prefer NTP (chronyd, systemd-timesyncd, or ntpd) to keep server clocks synchronized instead of manually setting time with date -s, which drifts and can cause certificate/logging issues.
• Standardize on ISO 8601 (date -I or date +%F) for logs and filenames — it sorts correctly as plain text and avoids locale-dependent month/day ordering ambiguity.
• When writing cross-platform scripts, test date-arithmetic commands on both GNU and BSD date if macOS support matters, since the flags for relative dates differ completely.

Real-world use cases

• Generating timestamped log file or backup archive names, e.g. backup_$(date +%Y%m%d).tar.gz.
• Comparing "now" against a deadline or expiration date in a deployment or cron script.
• Converting Unix epoch timestamps from application logs into human-readable dates for debugging.
• Adding a Date: header in raw email or HTTP messages using RFC 5322 format (date -R).

Common interview questions

• How would you generate a timestamp safe to use in a filename? date +"%Y%m%d_%H%M%S" — no spaces or slashes, sorts lexicographically.
• How do you get the current Unix epoch time, and convert it back? date +%s for the epoch; date -d @<epoch> to convert it back to a readable date.
• What's the difference between GNU date's -d and BSD date's -v? GNU date -d accepts free-text relative expressions like "+1 day" or "next friday"; BSD/macOS date -v uses adjustment flags like -v+1d instead, and the two are not interchangeable.