如何动态更改yii常量？

I have defined some constants in yii components.

component/Interst.php

class Interst {
    Const Rate = 15.6%
    Const Service_tax_rate = 14.5%
}

I am using this in my controllers like Interest::Service_tax_rate. I want to change this rate on 31st May 2016 to 15.0% this should be active from 12 AM (at night) of 31st. How can I code so that this would change automatically after 31st night?

I have tried this in Interest.php

if(date('d/m/Y H:i:s') == '31/05/2016 00:00:00'){
    Const Service_tax_rate = 15%;
}

but this doesn't work as a condition - it's causing an error

This should work:

if(time() >= strtotime('01/06/2016 00:00:00'))

Make sure you have all timestamps involved in the right format (e.g. timezone)
See: http://php.net/manual/en/function.strtotime.php
and: http://php.net/manual/en/function.time.php

And don't define Service_tax_rate as Const (if you want to change it)