From Kevin Paul Dupont of the Boston Globe:
The arbitration process, per the league’s collective bargaining agreement, allows clubs to revisit potential buyout plans for a 48-hour window after each arbitration award. Wideman’s arbitration hearing cannot be held earlier than July 20, nor later than Aug. 4.
Read more on this story and Dennis Wideman filing for artbitration here.
This allows teams more time to make decisions on their rosters. In the Bruins' case, Dupont points out that it allows for Boston management to decide about a Glen Murray buyout a little later on in the summer.
I've said this before, the NHL needs to move its arbitration hearings to July. Arbitration doesn't only affect players going to arbitration, it affects other players on the roster too.