GetBusinessTime : Function not available at client

Hello all,

I'm trying to get my function GetBusinessTime to works, but I always get that error message:

Error on line 3: Function not available at client.

I did try a really simple calculation to make sure if it was a calculation problem or more like a function problem, so here my simple calculation:

import System

static def GetAttributeValue(Incident):

  TimePeriod = Incident.GetBusinessTime(Incident.CreationDate, DateTime.UtcNow)

  return String.Format("{0} Hour(s), {1} Minute(s), {2} Second(s)", Math.Floor(TimePeriod.TotalHours), TimePeriod.Minutes, TimePeriod.Seconds)

Pretty much a copy-past from this document:

How to calculate the "business time" of an incident

I think it might be a problem with my service level management. My incident got a correct Response levels, and my response levels has a calendar link to it. So not sure why my function don't wanna work.

Can anyone help me?

Thanks!