I am having a hard time making a timer.

Right now I have a field "Duration" where the user enters how many minutes they need a room. 

When they go into the room, I have a drop down list of the Staus as Starting.   I then want to count down to how many minutes are left from the expected duration field.

 

This script that I write works, except that it does not dynamicly update.  e.g.  Room is needed for 120 min, on Submit, the Time Left returns 119 - and then stays at 199.  How do I make it continue to go down each time the page is refreshed?

 

Thank you.

 

 

 

//If room is starting, the start counting down from specified duration
if (input.Status  ==  "Starting")
{
    Start_Time = zoho.currenttime;
    input.Time_Left = (input.Duration  -  (zoho.currenttime - Start_Time));
}
else
{
    input.Time_Left = 9999;
}