How to replace a string at a particular position

The following is one option:

var myString = "Mar 16, 2010 00:00 AM";

myString = myString.substring(0, 13) + 
           "12" + 
           myString.substring(15, myString.length);

Note that if you are going to use this to manipulate dates, it would be recommended to use some date manipulation methods instead, such as those in DateJS.

A regex approach

"Mar 16, 2010 00:00 AM".replace(/(.{13}).{2}/,"$112")
Mar 16, 2010 12:00 AM

One option would be

>>> var test = "Mar 16, 2010 00:00 AM";
>>> test.replace(test.substring(13,15),"12")

if it is always 00: in hours,

you can just replace 00: with 12:

using replace() ,

if not u need find the indexOf the : character ,

and then replace 2 digit before with 12.

You can direclty use replace() method along with indexOf() of string in Javascript.

Another creative idea could be converting into Array, splice and convert it back to String.

let str = "Mar 16, 2010 00:00 AM";
let arr = str.split("");
arr.splice(13,2,"1","2");
str = arr.join("");