ORA-01722: invalid number

Normally, I have no problem renaming a column its original column name after some function. This time, it's posting 'invalid number', but only after I inserted the 'Order by'.

  1  Select To_Char(Trunc(Datetime) + (Trunc((Datetime - Trunc(Datetime)) *48) / 48), 'dd/Mm/Yy Hh:Mi') As Datetime
  2       , Count(*)
  3    From Ext_Ittbl1
  4   Group By To_Char(Trunc(Datetime) + (Trunc((Datetime - Trunc(Datetime)) *48) / 48), 'dd/Mm/Yy Hh:Mi')
  5*  Order By To_Char(Trunc(Datetime) + (Trunc((Datetime - Trunc(Datetime)) *48) / 48), 'dd/Mm/Yy Hh:Mi')
SQL> /
 Group By To_Char(Trunc(Datetime) + (Trunc((Datetime - Trunc(Datetime)) *48) / 48), 'dd/Mm/Yy Hh:Mi')
          *
ERROR at line 4:
ORA-01722: invalid number