Time Only

date +"%H:%M:%S"
# Output: 16:30:45

ISO 8601 (UTC)

date -u +"%Y-%m-%dT%H:%M:%SZ"
# Output: 2025-12-17T16:30:45Z

Unix Timestamp

date +%s
# Output: 1734451845

Human-Readable

date +"%d %B %Y"
# Output: 17 December 2025

Date Arithmetic

Yesterday

date -v-1d +"%Y-%m-%d"
# Output: 2025-12-16

N Days Ago

date -v-7d +"%Y-%m-%d"
# Output: 2025-12-10 (7 days ago)

Start of Month

date -v1d +"%Y-%m-%d"
# Output: 2025-12-01

Last Month

date -v-1m +"%Y-%m"
# Output: 2025-11

Comparison / Calculation

Days Between Dates

echo $(( ($(date -jf "%Y-%m-%d" "2025-12-17" +%s) - $(date -jf "%Y-%m-%d" "2025-12-10" +%s)) / 86400 ))
# Output: 7

Is Date Older Than N Days?

# Check if a date is older than 30 days
target_date="2025-11-01"
days_ago=$(( ($(date +%s) - $(date -jf "%Y-%m-%d" "$target_date" +%s)) / 86400 ))
echo $days_ago

Format Specifiers Reference

Specifier	Meaning	Example
`%Y`	Year (4 digit)	2025
`%m`	Month (01-12)	12
`%d`	Day (01-31)	17
`%H`	Hour (00-23)	16
`%M`	Minute (00-59)	30
`%S`	Second (00-59)	45
`%B`	Month name	December
`%A`	Day name	Wednesday
`%s`	Unix timestamp	1734451845
`%Z`	Timezone	GMT

Important Rules

Always fetch, never assume - Even if you think you know the date
Use appropriate format - ISO for logs, human-readable for display
Consider timezone - Use -u flag for UTC when needed
macOS uses BSD date - Syntax differs from GNU date (Linux)

Quick Reference

Need	Command
Current datetime	`date +"%Y-%m-%d %H:%M:%S"`
Current date	`date +"%Y-%m-%d"`
Current time	`date +"%H:%M:%S"`
Yesterday	`date -v-1d +"%Y-%m-%d"`
UTC timestamp	`date -u +"%Y-%m-%dT%H:%M:%SZ"`

datetime

Datetime Skill

Common Formats

Full Timestamp (ISO-like)

Date Only