Resolve "Average waiting time for time slot labs"

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Closes #363 (closed)

src/main/java/nl/tudelft/ewi/queue/model/Request.java

@@ -495,6 +495,12 @@ public class Request implements RecordsEvents<Request>, Serializable, Comparable
 		}
 
 		LocalDateTime processTime = getProcessingAt();
+		LocalDateTime startTime;
+
+		if (getLab().isSignOffIntervals())
+			startTime = slot.getOpensAt();
+		else
+			startTime = getCreatedAt();
 
 		//If there is no processing time and is not pending than the request is already handled
 		//so we take the handled time for calculation.
@@ -505,7 +511,7 @@ public class Request implements RecordsEvents<Request>, Serializable, Comparable
 			}
 		}
 
-		return ChronoUnit.MINUTES.between(getCreatedAt(), processTime);
+		return ChronoUnit.MINUTES.between(startTime, processTime);
 	}
 
 	/**