Just to clarify first of all: the datepicker format settings only apply to the frontend of the site. Within the event editor, the format is fixed (not so much to a US format, which would be more like 10/27/2014, but to the more generic yyyy-mm-dd format you are seeing). = OK
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That is strange and I can’t think why the date would “jump” or indeed why it would be locked in place. = Yes more then strange I would say annoying.
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It looks like you already ran through our standard troubleshooting steps, can you confirm if that enabled you to identify any conflicts? Did this strange behaviour persist even with a default, unmodified theme and no other plugins running? = YES