update dt870
set dt870.di062992 = (SELECT DATEADD(mm, DATEDIFF(mm,0,CONVERT(DATETIME, dt870.di028668, 104)), 0))
where dt870.di062992 is null or dt870.di062992 = CONVERT(DATETIME, '01.01.1970', 104);

update dt090
set dt090.di062993 = (SELECT DATEADD(mm, DATEDIFF(mm,0,CONVERT(DATETIME, dt090.OS_CREATED_ON, 104)), 0))
where dt090.di062993 is null or dt090.di062993 = CONVERT(DATETIME, '01.01.1970', 104);