Auto date/time field fill when another field is filled in

Per the title, I'm looking for a way to automatically add the date and time to a field at the moment another field is filled in.  I don't want to use stamps or signatures as i'd like to export these forms to excel in the future to log the data. Bit hard to explain but for example: User A enters initials in "IN1" field, as soon as they fill and exit the field, the date and time that the change was made is entered into a date/time field (ts1) to the right.  

I know this has to be done with a javascript, and I've got one that I believe should be working but I can't figure out how to get it running.  Script below:

// Declair Variables
var initials, timeStamp;

// Assign Variables
var initials = IN1
var timeStamp = ts1

if (timeStamp.value == null) {
if (initials.value != null) {
var d = Date(Date.now());
var ds = d.toString()
timestamp = ds.slice(1,24)
}
}

Thanks in advance!