CiviMail Send/Schedule form - Send Immediately checkbox is not cleared when an explicit date is set

The "Send Immediately" checkbox should be un-checked when user selects a data in "Schedule Mailing" date-picker. The existing javascript for this was broken when the date-picker fields were upgraded in 4.1. All date fields are now instantiated as a visible field with id of $fieldName_display, and a hidden $fieldName field. The javascript needs to be updated to reference the _display field id.