"Start date must be the same or later than Member since" triggered when dates are the same
I get this issue on my drupal/civicrm 5.10.3 instance. And I test it, and I've found it there too: https://civicrm.demo.civihosting.com
Using the same date for "start date" and "member since" fields should NOT trigger this error.
My workaround was change one of 2 dates, but this issue is a bug and an usability issue.
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