Odoo datetime default value for time 00:00:00

I have the following field in an Odoo V10 model

starting_date = fields.Datetime('Starting date', default='2018-01-01 00:00:00')

and I want the value of the default to take the current date but that the time is the 00:00:00 or any other fixed value to avoid that I put dates of type 11:27:23