🇩🇪Germany @Frando

Thanks for reporting, the mismatch between definition and value is certainly wrong. This would be a breaking change though, because currently, you can use the occurrence property values as DateTime objects. Do you know if SearchApi is able to work with DateTime objects directly (and is there a data definition type in Drupal for this)?

If not, I'll commit your patch once I do a new release and at the same time add the DateTime object as start_date and end_date properties (as with core datetime_range field item properties).