Linux Date Command: Complete Guide and Reference

Master the Linux date command with this comprehensive guide covering syntax, formatting, timezone operations, and practical scripting examples.

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Linux Date Command: Complete Guide and Reference

Table of Contents

- [Introduction](#introduction) - [Basic Syntax](#basic-syntax) - [Command Options](#command-options) - [Format Specifiers](#format-specifiers) - [Basic Usage Examples](#basic-usage-examples) - [Advanced Usage Examples](#advanced-usage-examples) - [Date Arithmetic](#date-arithmetic) - [Timezone Operations](#timezone-operations) - [Practical Applications](#practical-applications) - [Common Use Cases](#common-use-cases) - [Troubleshooting](#troubleshooting) - [Best Practices](#best-practices)

Introduction

The date command in Linux is a versatile utility used to display and manipulate system date and time information. It serves multiple purposes including displaying current date/time, formatting output in various ways, performing date calculations, and setting system time (with appropriate privileges). This command is essential for system administration, scripting, logging, and various automation tasks.

The date command reads the system clock and can display time in numerous formats, work with different timezones, and perform arithmetic operations on dates. It's particularly valuable in shell scripting for creating timestamps, scheduling tasks, and generating time-based file names.

Basic Syntax

`bash date [OPTION]... [+FORMAT] date [-u|--utc|--universal] [MMDDhhmm[[CC]YY][.ss]] `

Core Components

| Component | Description | Required | |-----------|-------------|----------| | date | Base command | Yes | | OPTION | Command line flags and options | No | | FORMAT | Output format specification preceded by + | No | | MMDDhhmm | Date/time setting format | No |

Command Options

Primary Options Table

| Option | Long Form | Description | Example | |--------|-----------|-------------|---------| | -d | --date=STRING | Display time described by STRING | date -d "tomorrow" | | -f | --file=DATEFILE | Process dates from file | date -f dates.txt | | -r | --reference=FILE | Display last modification time of FILE | date -r /etc/passwd | | -s | --set=STRING | Set system date/time | date -s "2024-01-01 12:00:00" | | -u | --utc, --universal | Display or set UTC time | date -u | | -R | --rfc-2822 | Output RFC 2822 compliant date | date -R | | -I | --iso-8601 | Output ISO 8601 date format | date -I | | | --help | Display help information | date --help | | | --version | Display version information | date --version |

Detailed Option Explanations

-d, --date=STRING This option allows you to display the date and time specified by STRING instead of the current date. STRING can be in various formats including relative dates like "yesterday", "next Monday", "2 hours ago", or absolute dates.

-f, --file=DATEFILE Processes multiple date strings from a file, with each line treated as a separate date specification. Useful for batch processing of dates.

-r, --reference=FILE Shows the last modification timestamp of the specified file rather than the current system time. This is equivalent to stat -c %y filename.

-s, --set=STRING Sets the system date and time. Requires root privileges. The STRING can be in various formats but must be unambiguous.

-u, --utc, --universal Forces the command to work with Coordinated Universal Time (UTC) instead of the local timezone.

Format Specifiers

Date Format Codes

| Code | Description | Example Output | |------|-------------|----------------| | %a | Abbreviated weekday name | Mon | | %A | Full weekday name | Monday | | %b | Abbreviated month name | Jan | | %B | Full month name | January | | %c | Complete date and time | Mon 01 Jan 2024 12:00:00 PM EST | | %d | Day of month (01-31) | 01 | | %D | Date in mm/dd/yy format | 01/01/24 | | %e | Day of month, space padded (1-31) | 1 | | %F | Full date in YYYY-MM-DD format | 2024-01-01 | | %g | 2-digit year for ISO week | 24 | | %G | 4-digit year for ISO week | 2024 | | %h | Same as %b | Jan | | %H | Hour in 24-hour format (00-23) | 12 | | %I | Hour in 12-hour format (01-12) | 12 | | %j | Day of year (001-366) | 001 | | %k | Hour in 24-hour format, space padded (0-23) | 12 | | %l | Hour in 12-hour format, space padded (1-12) | 12 | | %m | Month number (01-12) | 01 | | %M | Minute (00-59) | 00 | | %n | Newline character | | | %N | Nanoseconds (000000000-999999999) | 123456789 | | %p | AM or PM | PM | | %P | am or pm (lowercase) | pm | | %r | 12-hour time format | 12:00:00 PM | | %R | 24-hour time format HH:MM | 12:00 | | %s | Seconds since Unix epoch | 1704110400 | | %S | Second (00-60) | 00 | | %t | Tab character | | | %T | 24-hour time format HH:MM:SS | 12:00:00 | | %u | Day of week (1-7, Monday=1) | 1 | | %U | Week number, Sunday first (00-53) | 01 | | %V | ISO week number (01-53) | 01 | | %w | Day of week (0-6, Sunday=0) | 1 | | %W | Week number, Monday first (00-53) | 01 | | %x | Locale's date representation | 01/01/2024 | | %X | Locale's time representation | 12:00:00 PM | | %y | 2-digit year (00-99) | 24 | | %Y | 4-digit year | 2024 | | %z | Timezone offset from UTC | -0500 | | %Z | Timezone abbreviation | EST | | %% | Literal % character | % |

Time Format Examples Table

| Format String | Output Example | Use Case | |---------------|----------------|----------| | +%Y-%m-%d | 2024-01-01 | ISO date format | | +%d/%m/%Y | 01/01/2024 | European date format | | +%m/%d/%Y | 01/01/2024 | American date format | | +%Y%m%d_%H%M%S | 20240101_120000 | Filename timestamps | | +%A, %B %d, %Y | Monday, January 01, 2024 | Readable long format | | +%r | 12:00:00 PM | 12-hour time | | +%T | 12:00:00 | 24-hour time | | +%s | 1704110400 | Unix timestamp |

Basic Usage Examples

Displaying Current Date and Time

`bash

Display current date and time in default format

date

Output: Mon Jan 1 12:00:00 EST 2024

Display current date in ISO format

date -I

Output: 2024-01-01

Display current date and time in ISO format with time

date -Iseconds

Output: 2024-01-01T12:00:00-05:00

Display current UTC time

date -u

Output: Mon Jan 1 17:00:00 UTC 2024

`

Custom Format Examples

`bash

Display date in YYYY-MM-DD format

date +%Y-%m-%d

Output: 2024-01-01

Display time in HH:MM:SS format

date +%H:%M:%S

Output: 12:00:00

Display full weekday and month names

date +"%A, %B %d, %Y"

Output: Monday, January 01, 2024

Create a timestamp for filenames

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

Output: 20240101_120000

Display date with timezone information

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

Output: 2024-01-01 12:00:00 EST

`

Reference File Timestamps

`bash

Show last modification time of a file

date -r /etc/passwd

Output: Wed Dec 20 10:30:45 EST 2023

Show file timestamp in custom format

date -r /etc/passwd +"%Y-%m-%d %H:%M:%S"

Output: 2023-12-20 10:30:45

Compare file modification time with current time

echo "File: $(date -r /etc/passwd +%s)" echo "Now: $(date +%s)"

Output: File: 1703088645

Now: 1704110400

`

Advanced Usage Examples

Date Arithmetic and Relative Dates

`bash

Display tomorrow's date

date -d "tomorrow"

Output: Tue Jan 2 12:00:00 EST 2024

Display yesterday's date

date -d "yesterday"

Output: Sun Dec 31 12:00:00 EST 2023

Display date 1 week ago

date -d "1 week ago"

Output: Mon Dec 25 12:00:00 EST 2023

Display date 30 days from now

date -d "30 days"

Output: Wed Jan 31 12:00:00 EST 2024

Display next Monday

date -d "next Monday"

Output: Mon Jan 8 12:00:00 EST 2024

Display last Friday

date -d "last Friday"

Output: Fri Dec 29 12:00:00 EST 2023

`

Complex Date Calculations

`bash

Add 2 hours and 30 minutes to current time

date -d "2 hours 30 minutes"

Output: Mon Jan 1 14:30:00 EST 2024

Subtract 3 months from current date

date -d "3 months ago"

Output: Fri Oct 1 12:00:00 EDT 2023

Calculate date 90 days ago

date -d "90 days ago" +%Y-%m-%d

Output: 2023-10-03

Find what day of week a specific date was

date -d "2023-12-25" +%A

Output: Monday

Calculate days between two dates (using Unix timestamps)

date1=$(date -d "2024-01-01" +%s) date2=$(date -d "2024-01-31" +%s) echo $(( (date2 - date1) / 86400 )) days

Output: 30 days

`

Working with Specific Dates

`bash

Display specific date

date -d "2024-06-15"

Output: Sat Jun 15 00:00:00 EDT 2024

Display specific date and time

date -d "2024-06-15 14:30:00"

Output: Sat Jun 15 14:30:00 EDT 2024

Parse various date formats

date -d "June 15, 2024"

Output: Sat Jun 15 00:00:00 EDT 2024

date -d "15/06/2024"

Output: Sat Jun 15 00:00:00 EDT 2024

date -d "2024-06-15T14:30:00"

Output: Sat Jun 15 14:30:00 EDT 2024

`

Date Arithmetic

Arithmetic Operations Table

| Operation | Syntax Example | Description | |-----------|----------------|-------------| | Add days | date -d "+5 days" | Add 5 days to current date | | Subtract days | date -d "-10 days" | Subtract 10 days from current date | | Add weeks | date -d "+2 weeks" | Add 2 weeks to current date | | Add months | date -d "+3 months" | Add 3 months to current date | | Add years | date -d "+1 year" | Add 1 year to current date | | Add hours | date -d "+6 hours" | Add 6 hours to current time | | Add minutes | date -d "+45 minutes" | Add 45 minutes to current time | | Add seconds | date -d "+30 seconds" | Add 30 seconds to current time |

Complex Arithmetic Examples

`bash

Combine multiple operations

date -d "next Monday +2 weeks -1 day"

Output: Sun Jan 21 12:00:00 EST 2024

Calculate end of current month

date -d "$(date +%Y-%m-01) +1 month -1 day"

Output: Wed Jan 31 00:00:00 EST 2024

Calculate beginning of current year

date -d "$(date +%Y-01-01)"

Output: Mon Jan 1 00:00:00 EST 2024

Calculate last day of previous year

date -d "$(date +%Y-01-01) -1 day"

Output: Sun Dec 31 00:00:00 EST 2023

`

Business Day Calculations

`bash

Find next business day (skip weekends)

next_day=$(date -d "tomorrow") while [[ $(date -d "$next_day" +%u) -gt 5 ]]; do next_day=$(date -d "$next_day +1 day") done echo "Next business day: $next_day"

Calculate 30 business days from now (excluding weekends)

business_days=0 current_date=$(date) while [[ $business_days -lt 30 ]]; do current_date=$(date -d "$current_date +1 day") if [[ $(date -d "$current_date" +%u) -le 5 ]]; then ((business_days++)) fi done echo "30 business days from now: $current_date" `

Timezone Operations

Timezone Environment Variable

`bash

Display time in different timezones using TZ environment variable

TZ='America/New_York' date

Output: Mon Jan 1 12:00:00 EST 2024

TZ='Europe/London' date

Output: Mon Jan 1 17:00:00 GMT 2024

TZ='Asia/Tokyo' date

Output: Tue Jan 2 02:00:00 JST 2024

TZ='UTC' date

Output: Mon Jan 1 17:00:00 UTC 2024

`

Timezone Conversion Examples

`bash

Convert current time to different timezones

echo "Local: $(date)" echo "UTC: $(TZ='UTC' date)" echo "Tokyo: $(TZ='Asia/Tokyo' date)" echo "London: $(TZ='Europe/London' date)" echo "LA: $(TZ='America/Los_Angeles' date)"

Output:

Local: Mon Jan 1 12:00:00 EST 2024

UTC: Mon Jan 1 17:00:00 UTC 2024

Tokyo: Tue Jan 2 02:00:00 JST 2024

London: Mon Jan 1 17:00:00 GMT 2024

LA: Mon Jan 1 09:00:00 PST 2024

`

Common Timezone Codes Table

| Timezone | Code | Example Output | |----------|------|----------------| | Eastern Standard Time | EST | Mon Jan 1 12:00:00 EST 2024 | | Pacific Standard Time | PST | Mon Jan 1 09:00:00 PST 2024 | | Greenwich Mean Time | GMT | Mon Jan 1 17:00:00 GMT 2024 | | Central European Time | CET | Mon Jan 1 18:00:00 CET 2024 | | Japan Standard Time | JST | Tue Jan 2 02:00:00 JST 2024 | | Coordinated Universal Time | UTC | Mon Jan 1 17:00:00 UTC 2024 |

Practical Applications

Log File Management

`bash

Create log files with timestamps

echo "Application started" >> app_$(date +%Y%m%d).log

Rotate logs daily

log_file="application_$(date +%Y-%m-%d).log" echo "$(date): System backup completed" >> "$log_file"

Archive old logs (older than 30 days)

find /var/log -name "*.log" -mtime +30 -exec rm {} \;

Create backup with timestamp

backup_name="backup_$(date +%Y%m%d_%H%M%S).tar.gz" tar -czf "$backup_name" /important/data/ `

System Administration Scripts

`bash #!/bin/bash

System monitoring script with timestamps

echo "=== System Status Report ===" echo "Generated: $(date)" echo "Uptime: $(uptime)" echo "Disk Usage:" df -h echo "Memory Usage:" free -h echo "Current Users:" who echo "Report completed at: $(date)"

Schedule maintenance window

maintenance_start=$(date -d "next Sunday 2:00 AM") maintenance_end=$(date -d "next Sunday 4:00 AM") echo "Next maintenance window: $maintenance_start to $maintenance_end" `

File Organization

`bash

Organize files by date

for file in *.txt; do file_date=$(date -r "$file" +%Y-%m) mkdir -p "$file_date" mv "$file" "$file_date/" done

Clean up temporary files older than 7 days

find /tmp -type f -mtime +7 -delete

Create dated directories

mkdir "project_$(date +%Y%m%d)" mkdir "reports_$(date +%Y_%m)" mkdir "archive_$(date +%Y)" `

Common Use Cases

Database Operations

`bash

MySQL backup with timestamp

mysqldump -u user -p database > "db_backup_$(date +%Y%m%d_%H%M%S).sql"

PostgreSQL backup

pg_dump database > "pg_backup_$(date +%Y-%m-%d).sql"

Log database operations

echo "$(date '+%Y-%m-%d %H:%M:%S'): Database backup completed" >> db.log `

Web Server Log Analysis

`bash

Analyze today's Apache logs

grep "$(date +%d/%b/%Y)" /var/log/apache2/access.log | wc -l

Find errors from last hour

grep "$(date -d '1 hour ago' '+%d/%b/%Y:%H')" /var/log/apache2/error.log

Rotate web server logs

mv /var/log/nginx/access.log "/var/log/nginx/access_$(date +%Y%m%d).log" systemctl reload nginx `

Cron Job Scheduling

`bash

Add timestamp to cron job output

0 2 * /path/to/script.sh >> /var/log/cron_$(date +\%Y\%m).log 2>&1

Daily cleanup script

#!/bin/bash echo "$(date): Starting daily cleanup"

Cleanup operations here

echo "$(date): Daily cleanup completed"

Weekly report generation

0 0 0 /usr/local/bin/generate_report.sh "$(date +\%Y-\%W)" `

Troubleshooting

Common Issues and Solutions

Issue: Date command not found `bash

Check if date is installed

which date

Output: /bin/date

If not found, install coreutils package

On Debian/Ubuntu:

sudo apt-get install coreutils

On RHEL/CentOS:

sudo yum install coreutils `

Issue: Permission denied when setting date `bash

Setting system date requires root privileges

sudo date -s "2024-01-01 12:00:00"

Check current user privileges

id whoami `

Issue: Incorrect timezone display `bash

Check current timezone

timedatectl status cat /etc/timezone

Set timezone

sudo timedatectl set-timezone America/New_York

Verify timezone change

date `

Issue: Date parsing errors `bash

Invalid date format

date -d "invalid date"

Output: date: invalid date 'invalid date'

Use proper format

date -d "2024-01-01" date -d "January 1, 2024" date -d "01/01/2024" `

Debugging Date Issues

`bash

Verbose output for debugging

date --debug -d "next Friday"

Check system clock synchronization

timedatectl status

Verify NTP synchronization

ntpq -p

Check hardware clock

sudo hwclock --show `

Best Practices

Scripting Best Practices

`bash

Always use consistent date formats in scripts

LOG_DATE=$(date +%Y-%m-%d) TIMESTAMP=$(date +%Y%m%d_%H%M%S)

Use ISO format for portability

DATE_ISO=$(date -I) DATETIME_ISO=$(date -Iseconds)

Store timestamps in variables for consistency

START_TIME=$(date +%s)

... script operations ...

END_TIME=$(date +%s) DURATION=$((END_TIME - START_TIME)) echo "Script completed in $DURATION seconds" `

Performance Considerations

`bash

Avoid calling date multiple times in loops

current_date=$(date +%Y-%m-%d) for file in *.log; do mv "$file" "${current_date}_${file}" done

Use date arithmetic instead of multiple calls

Instead of:

tomorrow=$(date -d "$(date) +1 day")

Use:

tomorrow=$(date -d "tomorrow") `

Security Considerations

`bash

Validate date input in scripts

validate_date() { if date -d "$1" >/dev/null 2>&1; then return 0 else echo "Invalid date: $1" return 1 fi }

Use quotes around date strings

user_date="$1" if validate_date "$user_date"; then processed_date=$(date -d "$user_date" +%Y-%m-%d) fi `

Portability Tips

`bash

Use standard format specifiers for portability

Avoid GNU-specific extensions when possible

portable_date=$(date +%Y-%m-%d)

Test date operations on target systems

Some systems may have different date implementations

Use UTC for consistent behavior across systems

utc_timestamp=$(date -u +%s) `

The date command is an essential tool for Linux system administration and scripting. Its flexibility in formatting, timezone handling, and date arithmetic makes it invaluable for various tasks from simple timestamp generation to complex date calculations. Understanding its capabilities and proper usage patterns will significantly enhance your ability to work with dates and times in Linux environments.

Tags

  • Command Line
  • Linux
  • bash
  • scripting
  • system-administration

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