by Paul Nicholls.
This sounds suspiciously similar to MDL-39190 - David, are you/your clients by any chance using Safari? And does Daylight Savings Time start or end that weekend for you? There's a very annoying bug in Safari related to the way it handles DST, which can result in it changing the day erroneously - but (when I last looked into it) no sign of any intention to fix it at their end, leaving us to find ways to work around it. In the case of date pickers (as opposed to date/time pickers), the workaround is to use midday rather than midnight - but that obviously doesn't apply when you're actually trying to set the time to midnight...