Question

If I have the following string representing a time:

var s = "22:01:15";

I want to increment that time along with the system clock, but add to my existing value.

How do I continue this as if it was a real clock no matter what the actual computer clock time is?

Answer 1

Just take that time, parse it into a Date instance, and determine the difference between that and the system’s date. Then apply that offset.

1. Parse it:

   var parts = str.split(":");
   

2. Apply that to a date

   var dt = new Date();
   dt.setHours(parseInt(parts[0], 10));
   dt.setMinutes(parseInt(parts[1], 10));
   dt.setSeconds(parseInt(parts[2], 10));
   

3. Get the difference

   var diff = new Date().getTime() - dt.getTime();
   

4. Apply it any time you’re going to display the value

   var now = new Date(new Date().getTime() - diff);
   

5. Use the now instance to display

You do steps 1 – 4 once, at the outset; then repeat step 5 as necessary.

Live example