Let say the date field in stock.move is date_move and date field in stock.picking is date_picking . The field move_lines is a one2many from picking to stock move. So where ever the move_lines fields is using in the view add an attribute - context - to the view. For example

<field name="move_lines"  context="{'default_date_move':parent.date_picking}"/>

This will automatically select the date specified in the date_picking field to date_move when adding new stock moves.The problem is after adding the stock moves if anyone changes the date in date_picking it will not effect on the moves that are already added.