The UNIX timestamp that is used will be a string, but has to be a number.
/api/1451001600000
new Date('1451001600000').getTime() > 0
// false
The date format isn’t correct (note the order of day and month and the time).
let date_string = new Date(1451001600000).toDateString()
let result = date_string.split(" ")[0] + "," + date_string.substring(3) + " " + new Date().toTimeString(1451001600000).split("+")[0]
console.log("result", result)
// result Fri, Dec 25 2015 21:17:55 GMT
A request to /api/1451001600000 should return { unix: 1451001600000, utc: "Fri, 25 Dec 2015 00:00:00 GMT" }