How to Convert Epoch to ISODate

The great thing about recent versions of MongoDB is that they added a lot of new expressions for handling different types in aggregation.  The not so great thing is sometimes you still have to get things done in an older version that doesn't have the same capabilities.

A colleague asked me today how you can convert an epoch (number of milliseconds since 1970/1/1) to a proper ISODate format.  The answer comes from date math:

db.coll.aggregate({$addFields:{
      date:{$add:[
            ISODate("1970-01-01T00:00:00Z"),
            "$epoch"
      ]}
}})

Because $add supports adding numbers to date (treating the number as milliseconds) we get back a proper ISODate that the number in "epoch" field represents!