Similar to https://www.drupal.org/project/civicrm_entity/issues/2570319 → , but for activities.
i.e. when filtering by date (using a calendar and contextual filter), the activity_date_time field of the activity table is getting mangled with FROM_UNIXTIME.
Reverting back to 7.x-2.0 solves the issue.
Using CiviCRM 5.2.2.
As also noted here in a comment: https://civicrm.stackexchange.com/questions/6495/unable-to-filter-contri...
Closed: won't fix
2.1
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.