Power Query M Function to Convert Milliseconds to Date (Datetime)

convertToDate

(milliseconds as any, optional returnDateTime as nullable text) =>
let
    result = 
        if milliseconds <> null then
            let
                javaScriptBaseDate = #datetime(1970,1,1,0,0,0),
                //milliseconds = 1576483200000,
                isDateTime = if returnDateTime = "false" or returnDateTime = "" then false else true,
                seconds = milliseconds / 1000,
                durationInSeconds = #duration(0,0,0,seconds),
                dateRecord = DateTime.ToRecord(javaScriptBaseDate + durationInSeconds),
                finalDate = 
                    if isDateTime then 
                        #datetime(dateRecord[Year],dateRecord[Month],dateRecord[Day],dateRecord[Hour],dateRecord[Minute],dateRecord[Second])
                    else
                        #date(dateRecord[Year],dateRecord[Month],dateRecord[Day])
            in
                finalDate
        else
            null
in
    result