Datetime Skill
NEVER assume the current date or time. Always use bash commands to fetch it.
Common Formats
Full Timestamp (ISO-like)
date +"%Y-%m-%d %H:%M:%S"
# Output: 2025-12-17 16:30:45
Date Only
date +"%Y-%m-%d"
# Output: 2025-12-17
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" |
