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I'm looking for a formula to find out the difference between two dates (for example Business_date__c (date time field) and now() in hours, excluding weekends.

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Finding the Number of Business Hours Between Two Date/Times The formula for finding business hours between two Date/Time values expands on the formula for finding elapsed business days. It works on the same principle of using a reference Date/Time, in this case 1/8/1900 at 16:00 GMT (9 a.m. PDT), and then finding your Dates’ respective distances from that reference. The formula rounds the value it finds to the nearest hour and assumes an 8–hour, 9 a.m. – 5 p.m. work day.

ROUND( 8 * (
   ( 5 * FLOOR( ( DATEVALUE( date/time_1 ) - DATE( 1900, 1, 8) ) / 7) +
    MIN(5, 
     MOD( DATEVALUE( date/time_1 ) - DATE( 1900, 1, 8), 7) +
     MIN( 1, 24 / 8 * ( MOD( date/time_1 - DATETIMEVALUE( '1900-01-08 16:00:00' ), 1 ) ) )
    ) 
   )
 -
   ( 5 * FLOOR( ( DATEVALUE( date/time_2 ) - DATE( 1900, 1, 8) ) / 7) +
     MIN( 5,
      MOD( DATEVALUE( date/time_2 ) - DATE( 1996, 1, 1), 7 ) +
      MIN( 1, 24 / 8 * ( MOD( date/time_2 - DATETIMEVALUE( '1900-01-08 16:00:00' ), 1) ) )
    )
   ) 
  ), 
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You can change the eights in the formula to account for a longer or shorter work day. If you live in a different time zone or your work day doesn’t start at 9:00 a.m., change the reference time to the start of your work day in GMT. See A Note About Date/Time and Time Zones for more information.
You can Refer the same in Salesforce.com Help Documentation

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