There is currently an issue in core for
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Allow end date to be optional
Needs work
and I think that Time Range should also be compatible / have a patch to allow for the end time to be optional when the setting of Optional end date is enabled
Add functionality to set the end time as optional when Optional end date is enabled
Active
9.0
Code
Thanks for the idea. If upstream offers a merge proposal, there is a high probability that I will offer compatibility. If you provide a patch, I will review it as soon as possible. But now the core maintainers are still waiting for RTBC
https://www.drupal.org/project/drupal/issues/2827055#comment-14689319
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Add option to show only start or end date in the DateTime Range custom formatter
Needs review