Extrat Date in Date format from values stored in Database

Extrat Date in Date format from values stored in Database

We are using MS SQL server as repository to AppManage database.
 
AppManage is storing datetime as a bigint in SQL server vs a regular datetime field. Please recommend the function that can be used to convert this number as valid date as part of regular SQL queries.
 
Following is example of one of the fields that we need to see as Date in our SQL query results for custom reports
 
AM_ManagedObject COLLECTIONTIME = 1281061878306
 
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