Calendar losing events

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Not strictly a dev question but may have a dev answer.
The built-in Calendar & Events list appears to be losing dates.
I have a site set up with an events calendar. Events added are dropping off the editor and/or 'event list' blocks on the site.
Getting this working is critical to a client site that is very near completion.
I've used the calendar on many sites in the past and never experienced a problem.
To confirm the issue I just set up a clean demo install on concrete5.io - same issue.
On my client site, I have entries that show up on the event list block but are missing from the dashboard!?
The issue seems to mostly affect dates 5+ years in the future.
Is this a bug? am I going nuts?

jawbonelid