I've tried different solutions from odoo help and stackoverflow but couldn't get any much help.

my code:

start_date = fields.Date()
end_date = fields.Date()
total_months = fields.Char(compute='_total_months')

@api.multi
def _total_months(self):
    if self.start_date and self.end_date:
        for rec in self:
            s_date = dt.strptime(rec.start_date, '%d-%m-%Y')
            e_date = dt.strptime(rec.end_date, '%d-%m-%Y')
            rec.total_months = str((s_date - e_date).days)