I'm trying to compare number of business hours with number of hours (bh<48)
I have gone through few methods and tried the below method to get number of hours(difference between startdate and enddate :

    diff(BusinessHours.id, startdate, Datetime.now())<48)

But I'm constantly getting the same error for all the methods :
**"Method does not exist or incorrect signature: [BusinessHours].diff(Schema.SObjectField, Datetime, Datetime)"**

I dont understand this error as usage of methods are referred here 

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_businesshours.htm

Any suggestions are highly appreciated!
TIA