Why would the date value of the where statement change after save - publish and re-opening the workflow? The date I enter, regardless of the date/day of the week changes from anywhere from 1 day to 3-5 days before the date I originally entered.
I think I understand what you are mentioning, but have you attempted to remove the action and bring it back in? Also do you know the server setting for UTC time format. Shouldn't have an affect, but if you are seeing the values change, I'd be interested to see what the SharePoint date/time settings are as thats what is used when running workflows.