Here is an approach using str.slice(0, -n). Where n is the number of characters you want to truncate.

var str = 1437203995000;
str = str.toString();
console.log("Original data: ",str);
str = str.slice(0, -3);
str = parseInt(str);
console.log("After truncate: ",str);

Remove last 3 characters of a string

var str = '1437203995000';
str = str.substring(0, str.length-3);
// '1437203995'

Remove last 3 digits of a number

var a = 1437203995000;
a = (a-(a%1000))/1000;
// a = 1437203995

you just need to divide the Date Time stamp by 1000 like:

var a = 1437203995000;
a = (a)/1000;