Thanks Yenthe for your response here is my created fields

check_in_date = fields.Datetime(string="DateTime Opened", required=False,)
check_out_date = fields.Datetime(string="DateTime Closed", required=False,)
time_diff = fields.Float(string="Time Difference",  required=False )
sla_state = fields.Selection(string="SLA", selection=[('past_sla', 'Past SLA'), ('within_sla', 'Within SLA'), ], required=False, )
state = fields.Selection([

here is my function for calculating date difference

@api.onchange('check_in_date', 'check_out_date','time_diff')
def calculate_date(self):
    if self.check_in_date and self.check_out_date:
        print "++++++++++++++++++++++++++++++++++++++++++++++++++"
        print "++++++++++++++++++++++++++++++++++++++++++++++++++"
        t1 = datetime.strptime(str(self.check_in_date), '%Y-%m-%d %H:%M:%S')
        t2 = datetime.strptime(str(self.check_out_date), '%Y-%m-%d %H:%M:%S')
        t3 = t2 - t1
        self.time_diff = str(t3.days)
        print "++++++++++++++++++++++++++++++++++++++++++++++++++"
        print "++++++++++++++++++++++++++++++++++++++++++++++++++"

and here is my function for outputing the sla but its not giving me the result

@api.onchange('time_diff')
def get_time_diff(self):
    if self.time_diff >= 1:
        self.sla_state == 'Past SLA'
    else:
        self.sla_state == 'Within SLA'