Filter does not work with date range fields

Problem/Motivation

When I have a field with field type date_range and I add it to the index with SearchAPI type "Date range" then the content indexing fails with the following error:

[1:109] failed to parse field [node_field_date_range_single_range] of type [date_range] in document with id 'entity:node/2:en'. Preview of field's value: '1760042673'. [1:109] error parsing field [node_field_date_range_single_range], expected an object but got null for index: entity:node/2:en

When I add the start (value) and end (end_value) parts of the date range field one by one as separated index fields then the content indexing works. There is a valid document in the Elasticsearch index. Something like this:

{
  "node_field_date_range_single_end": [
    1760046350
  ],
  "node_field_date_range_single_start": [
    1760042673
  ],
  "search_api_id": [
    "entity:node/2:en"
  ],
  "search_api_datasource": [
    "entity:node"
  ],
  "search_api_language": [
    "en"
  ]
}

But the following views filter operators do not work:

<  Is less than
<= Is less than or equal to
=  Is equal to
!= Is not equal to
>= Is greater than or equal to
>  Is greater than

The use case. I have an "Event" content type with a date_range field, but I am not able to list all the "Upcoming events" or the "Past events".

Steps to reproduce

1. Follow the docs to Setup for working on an issue. Log in as an administrator.
2. Go to /admin/modules, and enable Core's Datetime Range (datetime_range module) and its dependencies.
3. Go to /admin/structure/types/add, and add an Event content type (machine name event).
4. Go to /admin/structure/types/manage/event/fields/add-field, click Date and time, enter Label = Event date (machine name field_event_date), Choose a field type = Date range, click the Continue button, then click the Save button to accept the defaults on the second page.
5. Go to /node/add/event, set Title = Workshops, Start date = 2026-04-07 09:00:00, End date = 2026-04-08 17:00:00, then click the Save button.
6. Go to /node/add/event, set Title = Sessions, Start date = 2026-04-09 09:30:00, End date = 2026-04-09 16:30:00, then click the Save button.
7. Go to /node/add/event, set Title = Sprints, Start date = 2026-04-10 09:45:00, End date = 2026-04-11 16:15:00, then click the Save button.
8. Go to /admin/config/search/search-api/add-index, set Index name = Events, Datasources = Content, Server = elasticsearch_server, then click the Save button.
9. Go to /admin/config/search/search-api/index/events/fields/add/nojs; and click the Add button in the Title (title) row and the Event date (field_event_date) row. Click the Expand button in the Event date (field_event_date) row, then click the Add button in the End date value (field_event_date:end_value) field. Then click the Done button. On the Manage fields for search index Events, in both the field_event_date row and field_event_date:end_value rows, set Type = Date. Click the Save changes button.
10. Go to /admin/config/search/search-api/index/events. Under Start indexing now, click the Index now button. Wait for the batch job to complete. You should see the Status message Successfully indexed 3 items.
11. Go to /admin/structure/views/add. Set View name = Event search (machine name event_search). Under View settings, Show Index Events sorted by: Unsorted. Under Page settings, check Create a page, and make sure that Path is set to event-search. Click the Save and edit button.
12. You are taken to the view/edit page at /admin/structure/views/view/event_search. Under Fields, remove all existing fields, then click the Add button, and check Title (indexed field) in the Content data-source category. Click the Add and configure fields button, then click the Apply button to accept the defaults on the second page. Under Filter criteria, click the Add button, and check Event date in the Content datasource category. Click the Add and configure filter criteria button, then check Expose this filter to visitors, to allow them to change it. Also check Expose operator. Click the Apply button. Don't forget to click Save on the view itself.
13. Go to /event-search. You see the Event search view you created. You see 3 results: Workshops, Sessions, and Sprints.
14. Set Event Date = Is less than 2026-04-09 09:30:00, then click the Apply button.
    Expected result (i.e.: with patch applied): You see 1 result: Workshops.
    Actual result (i.e.: before patch applied): You see 0 results.
15. Set Event Date = Is less than or equal to 2026-04-09 09:30:00, then click the Apply button.
    Expected result (i.e.: with patch applied): You see 2 results: Workshops, and Sessions.
    Actual result (i.e.: before patch applied): You see 0 results.
16. Note: Ignore the Is equal to option for now: it will be addressed in #3584265: Fix "= Is equal to" and "!= Is not equal to" operators on date fields. See #7 and #14 for details.
17. Set Event Date = Is not equal to 2026-04-09 09:30:00, then click the Apply button.
    Expected result (i.e.: with patch applied): You see 0 results and no Error message.
    Actual result (i.e.: before patch applied): You see 0 results, and the Error message Undefined operator "!=" for field "field_event_date" in filter condition..
    Note: Ignore the unexpected result (0 results) will be addressed in #3584265: Fix "= Is equal to" and "!= Is not equal to" operators on date fields. See #7 and #14 for details.
18. Set Event Date = Is greater than or equal to 2026-04-09 09:30:00, then click the Apply button.
    Expected result (i.e.: with patch applied): You see 2 results: Sessions, and Sprints.
    Actual result (i.e.: before patch applied): You see 3 results: Workshops, Sessions, and Sprints.
19. Set Event Date = Is greater than 2026-04-09 09:30:00, then click the Apply button.
    Expected result (i.e.: with patch applied): You see 1 result: Sprints.
    Actual result (i.e.: before patch applied): You see 3 results: Workshops, Sessions, and Sprints.
20. Set Event Date = Is between, Min = 2026-04-09 09:30:00, Max = 2026-04-10 10:00:00, then click the Apply button.
    Expected result (i.e.: with patch applied): You see 2 results: Sessions, Sprints.
    Actual result (i.e.: before patch applied): You see 3 results: Workshops, Sessions, and Sprints.
21. Set Event Date = Is not between, Min = 2026-04-09 09:30:00, Max = 2026-04-10 10:00:00, then click the Apply button.
    Expected result (i.e.: with patch applied): You see 1 result: Workshops.
    Actual result (i.e.: before patch applied): You see 3 results: Workshops, Sessions, and Sprints.
22. Note: Ignore the Negated regular expression option for now, it is out of scope: see #7 and #3577065: Drop support for not_regular_expression operator for search_api_date views filter.
23. Set Event Date = Is empty (NULL), then click the Apply button. You see 0 results.
    Note that a warning is logged, this will be addressed in #3584210: Irrelevant warnings when filter value is null.
24. Set Event Date = Is not empty (NOT NULL), then click the Apply button. You see 3 results: Workshops, Sessions, and Sprints.
    Note that a warning is logged, this will be addressed in #3584210: Irrelevant warnings when filter value is null.

Proposed resolution

Fix how we detect UNIX timestamps (Elasticsearch calls this epoch_second format) by checking if the value itself is an integer instead of assuming it must be an array containing integers.

Restore the tests mistakenly deleted in #3535733: Range Searches; make sure that we test BOTH ISO-8601 strings and UNIX integer timestamps.

Handle both variants of the "not equal" operator (Views uses != internally but Search API uses <> internally)

Remaining tasks

1. Write a patch - done by @sweetchuck in #2
2. Add tests - done by @mparker17 in #7
3. Community review and testing, move to RTBC if passes - done by @fathershawn in #18
4. Maintainer review and testing - @mparker17 is happy with it as of #16
5. Merge to 9.0.x - done by @mparker17 in #20
6. Merge to 8.0.x - done by @mparker17 in #22
7. Release 9.0.x - released in 9.0.0-alpha3 by @mparker17
8. Release 8.0.x - released in 8.0.0-alpha7 by @mparker17

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

Fixed an issue where the "Is less than", "Is less than or equal to", "Is greater than or equal to", "Is greater than", "Is between", and "Is not between" filters did not work for Date fields.

Note that Date field issues with "Is equal to" and "Is not equal to" will be addressed in a future issue.

Note that the warning mistakenly output with "Is empty (NULL)", and "Is not empty (NOT NULL)" will be addressed in a future issue.

Note that the "Negated regular expression" option for date fields is an issue with Search API, see #3577065: Drop support for not_regular_expression operator for search_api_date views filter.

Note that mapping Drupal core's Datetime Range fields to Elasticsearch Date Range date_range fields is blocked by Search API, see #3583991: Support core's date_range field type's end_value: as a work-around, index the start and end date as separate date fields.