mdediana opened a new pull request #9091:
URL: https://github.com/apache/airflow/pull/9091


   Currently Airflow can't execute some schedules, e.g., every 10 min between 
16:30 and 18:10. This type of schedule becomes possible if defined as multiple 
cron expressions.
   
   This PR is still a work in progress, meant for discussing the viability of 
such feature.
   
   Some points to consider:
   
   - The first commit shows what is possible with minimal code changes. The 
second commit goes further and replaces all uses of `croniter` with the new 
`Cron` class. Which one is preferable?
   
   - The second commit changes `normalized_schedule_interval` in DAG, moving 
the translation of cron presets (e.g `@daily`) into the `Cron` class? Is it a 
good idea? Maybe not, as at least one serialization test broke and had to be 
fixed, I imagine other things may also break.
   
   - How should this type of schedule interval be displayed on the web UI? For 
up to three or four elements it looks fine, but for example a schedule interval 
with 10 cron expressions in the list is difficult to show, starting with the 
`Schedule` column in the Home page.
   
   - At the moment the class lives in `utils/dates.py`, should it be somewhere 
else? Maybe create a `models/scheduleinterval.py`?
   
   - Missing documentation.
   
   ---
   Make sure to mark the boxes below before creating PR: [x]
   
   - [x] Description above provides context of the change
   - [x] Unit tests coverage for changes (not needed for documentation changes)
   - [x] Target Github ISSUE in description if exists
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message](http://chris.beams.io/posts/git-commit/)"
   - [ ] Relevant documentation is updated including usage instructions.
   - [x] I will engage committers as explained in [Contribution Workflow 
Example](https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst#contribution-workflow-example).
   
   ---
   In case of fundamental code change, Airflow Improvement Proposal 
([AIP](https://cwiki.apache.org/confluence/display/AIRFLOW/Airflow+Improvements+Proposals))
 is needed.
   In case of a new dependency, check compliance with the [ASF 3rd Party 
License Policy](https://www.apache.org/legal/resolved.html#category-x).
   In case of backwards incompatible changes please leave a note in 
[UPDATING.md](https://github.com/apache/airflow/blob/master/UPDATING.md).
   Read the [Pull Request 
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