Problems with how dates are stored in epoch format during form submissions

We have been occasionally seeing some bizarre occurrences of people submitting our forms and their date of birth somehow getting confused, then rejected by Salesforce during sync. Here is an example: 

Form Submission original data, notice the value "-61990768800" which i take it is epoch format. 

When the lead tried to sync, it failed with this error: "{FIELD_INTEGRITY_EXCEPTION: Date of Birth: invalid date: Thu Aug 06 00:00:00 GMT 5}"

The only way I have found to fix this sync error is to delete the date (which displays correctly in Marketo's interface as 4th of August 2005, or at least this is a DoB that makes sense to our business) and use the date picker to put it in again. 

While doing that, notice what the old stored value was: "0005-08-04"

When I input the value again using the date picker, this is what it looks like: "2005-08-04"

It seems like the year gets stored incorrectly, but somehow displays well on the interface. 

Funnily enough, that epoch value (if we are assuming correctly), translated to a date of birth that made no sense to us, since this is not our target audience: 

We haven't been able to figure out how this fails and why. Any thoughts?