python - Cannot convert datetime.timedelta() to Decimal django -

i substracting 1 date , result again substracting 8 hours. after converting result in decimal. @ time of saving object getting error

cannot convert datetime.timedelta(0, 21600) decimal 

code:

            fm.extra_hours = (fm.trip_closing_date - fm.trip_starting_date) - timedelta(hours=8)             #fm.extra_hours = time - timedelta(hours=8)             st = fm.extra_hours             print type(st)             print st.days             seconds = st.total_seconds()             hours = int(seconds // 3600)             print hours             print type(hours)             ext_hrs = float (hours)              print ext_hrs             print type(ext_hrs)             if st.days < 0:                 st = 0             else:                 y="xyz"              if ek>=1:                 fm.extra_kilometers = ek                 fm.extra_charge = ((((fm.vehical_type.per_extra_km_charge))*((fm.extra_kilometers)))+(((fm.vehical_type.per_extra_hour_charge))*(decimal(ext_hrs))))                 fm.pure_amount = ((((fm.total_kilometer))*((fm.vehical_type.per_extra_km_charge)))+(((fm.vehical_type.per_extra_hour_charge))*(decimal(ext_hrs))))                 fm.service_tax = (((fm.pure_amount)+(fm.driver_allownce_charge))*decimal(4.944))/decimal(100)                 fm.grand_total = ((fm.pure_amount)+(fm.driver_allownce_charge)+(fm.service_tax))                 fm.amount_payeble_now = (fm.grand_total)-(fm.less_adavance)                 fm.save() 

i dont know wrong doing? please me this.

full trace

    typeerror @ /home/save_local_invoice/ cannot convert datetime.timedelta(0, 21600) decimal request method: post request url:    http://localhost:8000/home/save_local_invoice/ django version: 1.7 exception type: typeerror exception value:     cannot convert datetime.timedelta(0, 21600) decimal exception location: c:\python27\lib\decimal.py in __new__, line 658 python executable:  c:\python27\python.exe python version: 2.7.8 python path:     ['c:\\users\\magowa\\desktop\\magowa',  'c:\\users\\magowa\\desktop\\magowa',  'c:\\windows\\system32\\python27.zip',  'c:\\python27\\dlls',  'c:\\python27\\lib',  'c:\\python27\\lib\\plat-win',  'c:\\python27\\lib\\lib-tk',  'c:\\python27',  'c:\\python27\\lib\\site-packages'] server time:    sat, 25 apr 2015 10:26:33 +0530  error on line fm.save()  

it seems you're trying save timedelta object decimal. not possible without stating how decimal should interpret object. guess want fm.my_decimal_field = my_time_delta_object.total_seconds().