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index.md job-management.md time-calculations.md

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Time Calculations

Cron time parsing, validation, and calculation utilities for working with cron expressions and scheduling. Provides both a comprehensive CronTime class and standalone utility functions for quick calculations.

Capabilities

CronTime Class

Handles cron expression parsing, validation, and next execution time calculations with timezone and DST handling.

/**
 * Class for parsing and calculating cron time schedules
 * Supports cron expressions, Date objects, and Luxon DateTime objects
 */
class CronTime {
  // Constructor overloads for timezone/utcOffset exclusivity
  constructor(
    source: string | Date | DateTime,
    timeZone?: string | null,
    utcOffset?: never
  );
  
  constructor(
    source: string | Date | DateTime,
    timeZone?: never,
    utcOffset?: number | null
  );

  constructor(
    source: string | Date | DateTime,
    timeZone?: string | null,
    utcOffset?: number | null
  );

  // Properties
  readonly source: string | DateTime;
  readonly timeZone?: string;
  readonly utcOffset?: number;
  readonly realDate: boolean;

  // Static methods
  static validateCronExpression(cronExpression: string): {
    valid: boolean;
    error?: CronError;
  };

  // Instance methods
  sendAt(): DateTime;
  sendAt(i: number): DateTime[];
  getTimeout(): number;
  toString(): string;
  toJSON(): string[];
  getNextDateFrom(start: Date | DateTime, timeZone?: string): DateTime;
}

Constructor Parameters:

source - Cron expression string, JavaScript Date, or Luxon DateTime object
timeZone - Timezone string (e.g., 'America/Los_Angeles'). Cannot be used with utcOffset
utcOffset - UTC offset in minutes. Cannot be used with timeZone

Usage Examples:

import { CronTime } from "cron";

// From cron expression
const cronTime = new CronTime('0 0 * * *'); // midnight daily

// With timezone
const cronTimeZone = new CronTime('0 0 * * *', 'America/New_York');

// With UTC offset (offset in minutes)
const cronOffset = new CronTime('0 0 * * *', null, -300); // UTC-5

// From Date object
const futureDate = new Date('2024-12-31T23:59:59Z');
const dateTime = new CronTime(futureDate);

// From Luxon DateTime
import { DateTime } from 'luxon';
const luxonTime = DateTime.now().plus({ hours: 1 });
const cronFromLuxon = new CronTime(luxonTime);

Static Validation

Validate cron expressions without creating a CronTime instance.

/**
 * Validate a cron expression
 * @param cronExpression - Cron expression string to validate
 * @returns Object with validation result and optional error
 */
static validateCronExpression(cronExpression: string): {
  valid: boolean;
  error?: CronError;
};

Usage Examples:

// Valid expressions
let result = CronTime.validateCronExpression('0 0 * * *');
console.log(result.valid); // true

result = CronTime.validateCronExpression('*/5 * * * *');
console.log(result.valid); // true

// Invalid expression
result = CronTime.validateCronExpression('invalid');
console.log(result.valid); // false
console.error(result.error?.message); // Error details

// Check before creating CronTime
const expression = '0 0 29 2 *'; // Feb 29th
const validation = CronTime.validateCronExpression(expression);
if (validation.valid) {
  const cronTime = new CronTime(expression);
  console.log('Next execution:', cronTime.sendAt().toISO());
} else {
  console.error('Invalid cron expression:', validation.error?.message);
}

Time Calculation Methods

Methods for calculating execution times and intervals.

/**
 * Calculate next execution time
 * @returns Luxon DateTime object for next execution
 */
sendAt(): DateTime;

/**
 * Calculate multiple future execution times
 * @param i - Number of future executions to calculate
 * @returns Array of Luxon DateTime objects
 */
sendAt(i: number): DateTime[];

/**
 * Get milliseconds until next execution
 * Can return negative value if calculation takes too long
 * @returns Number of milliseconds until next execution
 */
getTimeout(): number;

/**
 * Get next execution time from a specific start point
 * @param start - Starting date/time for calculation
 * @param timeZone - Optional timezone override
 * @returns Luxon DateTime object for next execution
 */
getNextDateFrom(start: Date | DateTime, timeZone?: string): DateTime;

Usage Examples:

const cronTime = new CronTime('0 */4 * * *'); // every 4 hours

// Next execution
const next = cronTime.sendAt();
console.log('Next execution:', next.toISO());

// Next 5 executions
const nextFive = cronTime.sendAt(5);
nextFive.forEach((date, i) => {
  console.log(`Execution ${i + 1}:`, date.toISO());
});

// Milliseconds until next execution
const timeout = cronTime.getTimeout();
console.log('Execute in', timeout, 'milliseconds');

// Calculate from specific start time
const startTime = new Date('2024-01-01T00:00:00Z');
const nextFromStart = cronTime.getNextDateFrom(startTime);
console.log('Next from start:', nextFromStart.toISO());

// Handle negative timeout (missed execution)
if (timeout < 0) {
  console.warn('Execution was missed by', Math.abs(timeout), 'ms');
}

String Representation Methods

Methods for converting CronTime back to string representations.

/**
 * Convert to cron string representation
 * @returns Cron expression string
 */
toString(): string;

/**
 * Get JSON representation of parsed cron components
 * @returns Array of strings representing each cron field
 */
toJSON(): string[];

Usage Examples:

const cronTime = new CronTime('*/15 9-17 * * 1-5');

// Get cron string
console.log(cronTime.toString()); // "*/15 9-17 * * 1-5"

// Get JSON representation
console.log(cronTime.toJSON());
// ["*/15", "9-17", "*", "*", "1-5"]

// Reconstruct from JSON
const jsonRep = cronTime.toJSON();
const reconstructed = jsonRep.join(' ');
console.log('Reconstructed:', reconstructed); // "*/15 9-17 * * 1-5"

Standalone Utility Functions

Quick utility functions for common time calculations without creating CronTime instances.

/**
 * Get next execution time for a cron expression
 * @param cronTime - Cron expression, Date, or DateTime
 * @returns Luxon DateTime object for next execution
 */
function sendAt(cronTime: string | Date | DateTime): DateTime;

/**
 * Get milliseconds until next execution
 * @param cronTime - Cron expression, Date, or DateTime
 * @returns Number of milliseconds until next execution
 */
function timeout(cronTime: string | Date | DateTime): number;

/**
 * Validate a cron expression
 * @param cronExpression - Cron expression string to validate
 * @returns Object with validation result and optional error
 */
function validateCronExpression(cronExpression: string): {
  valid: boolean;
  error?: CronError;
};

Usage Examples:

import { sendAt, timeout, validateCronExpression } from "cron";

// Quick next execution time
const nextDaily = sendAt('0 0 * * *');
console.log('Next daily execution:', nextDaily.toISO());

// Quick timeout calculation
const dailyTimeout = timeout('0 0 * * *');
console.log('Daily job executes in:', dailyTimeout, 'ms');

// Quick validation
const isValid = validateCronExpression('0 0 * * *');
console.log('Valid expression:', isValid.valid);

// With Date objects
const futureDate = new Date(Date.now() + 3600000); // 1 hour from now
const dateTimeout = timeout(futureDate);
console.log('Date execution in:', dateTimeout, 'ms');

// Validation in conditionals
if (validateCronExpression(userInput).valid) {
  console.log('Starting job with expression:', userInput);
  // Create and start job
} else {
  console.error('Invalid cron expression provided');
}

Cron Expression Format

The cron expressions support enhanced 6-field format with second precision:

field          allowed values
-----          --------------
second         0-59
minute         0-59
hour           0-23
day of month   1-31
month          1-12 (or names: jan-dec)
day of week    0-7 (0 or 7 is Sunday, or names: sun-sat)

Expression Examples

// Standard patterns
'0 0 * * *'        // Every day at midnight
'*/5 * * * *'      // Every 5 minutes
'0 0 */2 * *'      // Every 2 days at midnight
'0 9-17 * * 1-5'   // Every hour from 9 AM to 5 PM, Monday to Friday

// With seconds
'*/30 * * * * *'   // Every 30 seconds
'0 */15 * * * *'   // Every 15 minutes (at :00 seconds)

// Using names
'0 0 * * mon-fri'  // Weekdays at midnight
'0 0 1 jan *'      // New Year's Day

// Preset expressions
'@yearly'   // 0 0 0 1 1 *
'@monthly'  // 0 0 0 1 * *
'@weekly'   // 0 0 0 * * 0
'@daily'    // 0 0 0 * * *
'@hourly'   // 0 0 * * * *
'@minutely' // 0 * * * * *
'@secondly' // * * * * * *

Timezone Handling

// System timezone (default)
const localTime = new CronTime('0 0 * * *');

// Specific timezone
const nyTime = new CronTime('0 0 * * *', 'America/New_York');
const utcTime = new CronTime('0 0 * * *', 'UTC');

// UTC offset (in minutes)
const offsetTime = new CronTime('0 0 * * *', null, -300); // UTC-5

// DST handling is automatic
const dstTime = new CronTime('0 2 * * *', 'America/New_York');
// Automatically handles spring forward and fall back

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Files