CronCalc: A Ruby gem for calculating CRON job occurrences. With this gem, you can easily determine when a cron job will occur by providing a cron expression. Key features include the ability to calculate occurrences:

• Within a specified period: Find out all the times your cron job will run during a particular timeframe.
• After a given date: Determine the next set of occurrences after a specific starting point.
• Before a given date: Discover when your cron job ran or would have run before a certain date.

This tool can be used for scheduling, forecasting, and analyzing tasks in systems that use cron for job scheduling. You can also read the article about CronCalc here.

Install the gem by executing:

$ gem install cron_calc

After installing cron_calc you can initialize CronCalc with the CRON string.

    require 'cron_calc'
    cron_calc = CronCalc.new('5 5 * * *')

Now, you can use one of three methods #in, #next, #last to determine cron job occurrencies within specified period.

Calculates cron job occurrences within a given time period.
Parameters:

• period - a Range object defining the start and end times for the calculation.\

    period = Time.new(2024, 1, 1, 0, 0)..Time.new(2024, 1, 4, 0, 0)
    cron_calc.in(period)

    # => [2024-01-01 05:05:00 +0100, 2024-01-02 05:05:00 +0100, 2024-01-03 05:05:00 +0100]

Calculates the next 'n' occurrences of the cron job from a given start time.
Parameters:

• count - (optional, Integer) The number of occurrences to calculate. Defaults to 1.
• after: - (optional, Time, keyword argument) The start time from which to calculate occurrences. If not provided, defaults to the current time (Time.now).
• max_years - (optional, Integer, keyword argument) The maximum number of years to search for future occurrences. Defaults to 5.

    cron_calc.next
    # => [2023-12-20 05:05:00 +0100]

    cron_calc.next(3)
    # => [2023-12-20 05:05:00 +0100, 2023-12-21 05:05:00 +0100, 2023-12-22 05:05:00 +0100]

    cron_calc.next(2, after: Time.new(2024, 1, 1, 0, 0))
    # => [2024-01-01 05:05:00 +0100, 2024-01-02 05:05:00 +0100]

Calculates the last 'n' occurrences of the cron job until a given end time.
Parameters:

• count - (optional, Integer) The number of occurrences to calculate. Defaults to 1.
• before: - (optional, Time, keyword argument) The end time from which to calculate past occurrences. If not provided, defaults to the current time (Time.now).
• max_years - (optional, Integer, keyword argument) The maximum number of years to search backward for past occurrences. Defaults to 5.

    cron_calc.last
    # => [2023-12-19 05:05:00 +0100]

    cron_calc.last(4, before: Time.new(2024, 1, 1, 0, 0))
    # => [2023-12-31 05:05:00 +0100, 2023-12-30 05:05:00 +0100, 2023-12-29 05:05:00 +0100, 2023-12-28 05:05:00 +0100]

    # You can omit the count parameter
    CronCalc.new('5 5 */5 * SUN').last(before: Time.new(2020, 1, 1, 0, 0))
    # => [2019-12-01 05:05:00 +0100]

    # You can combine ',' and '-'
    CronCalc.new('5 5 5-7,10 FEB *').next(5)
    # => [2024-02-05 05:05:00 +0100, 2024-02-06 05:05:00 +0100, 2024-02-07 05:05:00 +0100, 2024-02-10 05:05:00 +0100, 2025-02-05 05:05:00 +0100]

    # You can use predefined definitions like @daily, @monthly, etc.
    CronCalc.new('@monthly').next(3, after: Time.new(2024, 1, 1, 0, 0))
    # => [2024-01-01 00:00:00 +0100, 2024-02-01 00:00:00 +0100, 2024-03-01 00:00:00 +0100]

    # I want to know when the next 10 Friday the 13ths will be!
    CronCalc.new('0 0 13 * FRI').next(10)
    # => 
    # [2024-09-13 00:00:00 +0200,
    #  2024-12-13 00:00:00 +0100,
    #  2025-06-13 00:00:00 +0200,
    #  2026-02-13 00:00:00 +0100,
    #  2026-03-13 00:00:00 +0100,
    #  2026-11-13 00:00:00 +0100,
    #  2027-08-13 00:00:00 +0200,
    #  2028-10-13 00:00:00 +0200]

Bug reports and pull requests are welcome on GitHub at https://github.com/mizinsky/cron_calc.

The gem is available as open source under the terms of the MIT License.