Support `L` notation in `Day-of-week`

[a5-stable]

Now we can use L at Day-of-month like the following example;

  cron_parser = CronParser.new('0 9 L * *')

  time = Time.local(2022, 1, 16, 12, 0)
  cron_parser.next(time)
  # => 2022-01-31 09:00

I'd like to support L notation at Day-of-week in order to specify the last day of the week.

  cron_parser = CronParser.new('0 9 * * 4L')

  time = Time.local(2022, 1, 16, 12, 0)
  cron_parser.next(time)
  # => 2022-01-27 09:00

  # Last Friday in this month!

[RishiKumarRay]

So do we have to add L after 9 right?

[a5-stable]

@RishiKumarRay
Greetings, thank you for supporting.

add L after 9

Are you talking about the notation of 0 9 L * *, right?
0 9 L * * has L in the Day-of-month field and we already support this kind of notation.
（this specifies the last day of the month）

What I'd like to support is something like 0 9 * * 4L and this has L in the Day-of-week field.
This will specify the last Friday of each month.

I hope that answers your questions.

[RishiKumarRay]

@a5-stable do you have any references for solving this issue

[a5-stable]

@RishiKumarRay
Yes, I am thinking about something like AWS Cron Expressions used by Amazon CloudWatch.
https://docs.aws.amazon.com/AmazonCloudWatch/latest/events/ScheduledEvents.html

The next example creates a rule that is triggered at 10:15am UTC on the last Friday of each month during the years 2002 to 2005.

aws events put-rule --schedule-expression "cron(15 10 ? * 6L 2002-2005)" --name MyRule3

aws cron structure is a little different from what we're dealing with but the basic idea is in common.

[a5-stable]

still help wanted