[SalesForce] Can i create a formula field to calculate the time in business hours between Case created date to now/Closed date

This is sample trigger for calculating business hours excluding weekends and holidays:-

trigger calcBusinessHours on Submitted_Order__c (before insert, before update) {

    // Assumes the BSC work hours are the default for the Org
    BusinessHours stdBusinessHours = [select id from businesshours where isDefault = true];

        for (Submitted_Order__c so : Trigger.new) {
            if ((so.WB_Submitted__c != NULL) && (stdBusinessHours != NULL)) {
            // BSC works 11.5 hours / day (8:00 AM - 7:30 PM, M-F). Our SLA is 2-days (23 business hours)
            so.SLA_Due_Date__c = BusinessHours.addGmt (stdBusinessHours.id, so.WB_Submitted__c, 23 * 60 * 60 * 1000L);
            }
      }
 }

Write a trigger for calculating due date excluding weekends and holidays. The formula provided by you excludes weekends but not holidays

You are correct - Looking at the SFDC Object reference, Case.closedDate is a DateTime field so line 2 and line 4 will break

Looking at line 2 and line 4 - these are manipulating days, not seconds so I'd rewrite line 2 and line 4 as:

Line 2

( 5 * FLOOR( ( DATEVALUE(ClosedDate) - DATE( 1900, 1, 8) ) / 7) +

Line 4

MOD( DATEVALUE(ClosedDate) - DATE( 1900, 1, 8), 7) +

You should go to the doc and use the feedback form to point this out to SFDC