date

Overview

The date command displays or sets the system date and time. It’s essential for timestamping, scheduling, and time-based operations in scripts and system administration.

Syntax

date [options] [+format]
date [options] [MMDDhhmm[[CC]YY][.ss]]

Common Options

Option	Description
-d string	Display time described by string
-f file	Process dates from file
-r file	Display file’s last modification time
-s string	Set system date/time
-u	Display/set UTC time
--iso-8601	ISO 8601 format
--rfc-3339	RFC 3339 format

Format Specifiers

Format	Description	Example
%Y	Year (4 digits)	2024
%y	Year (2 digits)	24
%m	Month (01-12)	03
%B	Month name	March
%b	Month abbreviation	Mar
%d	Day of month	15
%A	Day name	Monday
%a	Day abbreviation	Mon
%H	Hour (00-23)	14
%I	Hour (01-12)	02
%M	Minute	30
%S	Second	45
%p	AM/PM	PM
%Z	Timezone	EST
%s	Seconds since epoch	1710504645

Key Use Cases

1. Display current date/time
2. Format timestamps
3. Calculate date differences
4. Log file naming
5. Script timing

Examples with Explanations

Example 1: Current Date and Time

date

Output: Mon Mar 15 14:30:45 EST 2024

Example 2: Custom Format

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

Output: 2024-03-15 14:30:45

Example 3: ISO Format

date --iso-8601

Output: 2024-03-15

Example 4: Specific Date

date -d "2024-12-25"

Output: Wed Dec 25 00:00:00 EST 2024

Date Arithmetic

1. Add days:

   date -d "+7 days"
   date -d "next week"

2. Subtract time:

   date -d "-1 month"
   date -d "yesterday"

3. Specific calculations:

   date -d "2024-01-01 +100 days"

Common Usage Patterns

1. Timestamp for logs:

   echo "$(date): Process started" >> log.txt

2. Backup file naming:

   cp file.txt "file_$(date +%Y%m%d_%H%M%S).txt"

3. Age calculation:

   date -d "1990-01-01" +%s  # Birth timestamp

File Timestamps

1. Show file modification time:

   date -r filename

2. Compare file ages:

   if [ $(date -r file1 +%s) -gt $(date -r file2 +%s) ]; then
       echo "file1 is newer"
   fi

Time Zones

1. UTC time:

   date -u

2. Specific timezone:

   TZ='America/New_York' date
   TZ='Europe/London' date

3. Convert timezone:

   date -d "2024-03-15 14:30:00 UTC" "+%Y-%m-%d %H:%M:%S %Z"

Performance Analysis

• Very fast operation
• Minimal system resources
• Good for frequent calls
• Efficient timestamp generation
• Low overhead

Related Commands

• timedatectl - System time control
• hwclock - Hardware clock
• cal - Calendar display
• uptime - System uptime
• sleep - Delay execution

Best Practices

1. Use consistent date formats
2. Consider timezone implications
3. Use epoch time for calculations
4. Validate date inputs
5. Handle leap years properly

Scripting Applications

1. Log rotation by date:

   #!/bin/bash
   LOG_DATE=$(date +%Y%m%d)
   mv app.log "app_${LOG_DATE}.log"

2. Backup automation:

   BACKUP_DIR="/backup/$(date +%Y/%m/%d)"
   mkdir -p "$BACKUP_DIR"

3. Performance timing:

   START_TIME=$(date +%s)
   # ... operations ...
   END_TIME=$(date +%s)
   DURATION=$((END_TIME - START_TIME))
   echo "Operation took $DURATION seconds"

Date Parsing

1. Parse various formats:

   date -d "March 15, 2024"
   date -d "15/03/2024"
   date -d "2024-03-15T14:30:00"

2. Relative dates:

   date -d "next Monday"
   date -d "last Friday"
   date -d "2 weeks ago"

Integration Examples

1. With find for file operations:

   find /logs -name "*.log" -newermt "$(date -d '7 days ago')"

2. Cron job scheduling:

   # Run only on weekdays
   if [ $(date +%u) -le 5 ]; then
       run_weekday_job
   fi

3. System monitoring:

   echo "$(date): CPU usage $(top -bn1 | grep "Cpu(s)" | awk '{print $2}')" >> monitor.log

Epoch Time

1. Current epoch:

   date +%s

2. Convert from epoch:

   date -d @1710504645

3. Date difference in seconds:

   START=$(date -d "2024-01-01" +%s)
   END=$(date -d "2024-12-31" +%s)
   DIFF=$((END - START))
   DAYS=$((DIFF / 86400))

Formatting Examples

1. Log format:

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

2. Filename safe:

   date "+%Y%m%d_%H%M%S"

3. Human readable:

   date "+%A, %B %d, %Y at %I:%M %p"

Troubleshooting

1. Timezone confusion
2. Daylight saving time issues
3. Leap year calculations
4. Date format parsing errors
5. System clock synchronization

Security Considerations

1. Validate date inputs
2. Be aware of timezone attacks
3. Use NTP for time synchronization
4. Log timestamp integrity
5. Handle time-based race conditions

Advanced Usage

1. Week calculations:

   date +%V  # ISO week number
   date +%U  # Week number (Sunday start)
   date +%W  # Week number (Monday start)

2. Day of year:

   date +%j  # Day of year (001-366)

3. Quarter calculation:

   MONTH=$(date +%m)
   QUARTER=$(((MONTH - 1) / 3 + 1))
   echo "Q$QUARTER"