UTC Time Format from API call

[catremor]

I have configured a source in Power Query Editor to import data in which the times are in UTC milliseconds. See sample below.

This results in Power Query converting this to scientific notation. See sample below.

I have attempted creating a Custom Column using various formulas that I have found but they have only resulted in 'Error' messages. 

Is there a way to prevent this conversion to scientific notation during import? Or, once imported, a formula that can be used to convert this to a readable date format?

[#2]

@catremor 

To convert from Unix time, which is number of milliseconds (in your case , could be seconds) from start of 01 Jan 1700 UTC

to common one

you may use something like

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    ReplaceStart = Table.ReplaceValue(
        Source,
        each [Start],
        each #datetime(1970, 1, 1, 0, 0, 0) + #duration(0, 0, 0, [Start]/1000),
        Replacer.ReplaceValue,{"Start"}
    ),
    ReplaceEnd = Table.ReplaceValue(
        ReplaceStart,
        each [End],
        each #datetime(1970, 1, 1, 0, 0, 0) + #duration(0, 0, 0, [End]/1000),
        Replacer.ReplaceValue,{"End"}
    )

in
    ReplaceEnd

 To adjust time zone use second parameter in #duration(), e.g. -3 for UTC-3