The fix is to change things like this: targetEndDate = new Date(); targetEndDate.setFullYear( endDate.getFullYear() ); targetEndDate.setMonth( endDate.ge

The fix is to change things like this: targetEndDate = new Date(); targetEndDate.setFullYear( endDate.getFullYear() ); targetEndDate.setMonth( endDate.getMonth() ); targetEndDate.setDate( endDate.getDate() ); to targetEndDate = new Date(); targetEndDate.setFullYear( endDate.getFullYear(), endDate.getMonth(), endDate.getDate()); That works because setFullYear has a three argument form that takes the year, month, and date avoiding the inconsistency that can arise by setting those one at a time. But you know what else has a three argument for that take the year, month, and date? The Date constructor. So why not fix it like this? targetEndDate = new Date( endDate.getFullYear(), endDate.getMonth(), endDate.getDate()); Using the empty constructor would initial the object with the current date and time, but they promptly overwrite those. The Date construction also has a form that takes another Date object, so I wonder if they could have simple used: targetEndDate = new Date(endDate);