Recalc of date field decending from previous day, Sql Server

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CREATE TABLE grad
(
  CityName varchar(9), 
  data1 decimal(8, 3),
  data2 decimal(8, 4),
  "date" datetime
);

INSERT INTO grad (CityName, data1, data2, "date") VALUES
 ('Mumbai', 1.234, 2.3456, '2018-01-20 14:00:00')
,('Mumbai', 6.234, 2.3456, '2018-01-20 20:00:00')
,('Mumbai', 1.234, 2.3456, '2018-01-21 14:00:00')
,('Mumbai', 2.234, 2.3456, '2018-01-21 20:00:00')
,('Mumbai', 1.234, 2.3456, '2018-01-22 02:00:00')
,('Mumbai', 1.234, 2.3456, '2018-01-21 02:00:00')
,('Mumbai', 8.234, 2.3456, '2018-01-19 03:00:00')
,('Mumbai', 3.334, 2.3456, '2018-01-19 14:00:00')
,('Mumbai', 8.214, 2.3456, '2018-01-19 16:00:00')
,('Mumbai', 19.234, 2.3456, '2018-01-18 14:00:00')
;

select CityName, data1, data2 
, dateadd(hour, 
         (((row_number() over (partition by CityName order by "date")-1)%4)*6)
       -((floor((row_number() over (partition by CityName order by "date")-1)/4)+1)*24),
         cast(cast(current_timestamp as Date) as datetime))  as "newdate"
from grad as t
where CityName = 'Mumbai'
order by t."date"

View schema
Execution time: 0,02 sec, rows selected: 10, rows affected: 10, absolute service time: 0,16 sec