SQL Server 2016

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SELECT DATEADD(second, 1526628000258 / 1000, '1970-01-01 01:00:00.000');

(No column name)

2018-05-18 08:20:00.000

SELECT DATEADD(millisecond, 1526628000258 % 1000,DATEADD(second, 1526628000258 / 1000, '1970-01-01 01:00:00.000'));

(No column name)

2018-05-18 08:20:00.257

SELECT DATEADD(millisecond, 1526628000258 % 1000, CAST(DATEADD(second, 1526628000258 / 1000, '1970-01-01 01:00:00.000') AS DATETIME2));

(No column name)

2018-05-18 08:20:00.2580000