How do I detect / retrieve the silicon revision of a dsPIC?
According the the errata sheet http://ww1.microchip.com/downloads/en/DeviceDoc/80372B.pdf:


  dsPIC33FJ32MC302/304, dsPIC33FJ64MCX02/X04
  and dsPIC33FJ128MCX02/X04 Rev. A1/A2/A3 silicon
  is identified by performing a “Reset and Connect”
  operation to the device using MPLAB® ICD 2 with
  MPLAB IDE v7.40 or later. The output window will show
  a successful connection to the device specified in
  Configure>Select Device. The resulting DEVREV
  register values for Rev. A1/A2/A3 silicon are 0x3001,
  0x3002 and 0x3003, respectively.


More recent, the family errata sheet http://ww1.microchip.com/downloads/en/DeviceDoc/80442H.pdf:
Has tables listing device ID and revision codes through revision A5 and states:


  The silicon revision level can be identified using the
  current version of MPLAB® IDE and Microchip’s
  programmers, debuggers and emulation tools, which
  are available at the Microchip corporate web site
  (www.microchip.com).


I don't have your chip, but with a PIC32MX360F512L connected to an ICD3, I get the revision from MPLAB-X by selecting "Debug - Discreet Debugger Operation - Launch Debugger". On the ICD3 window after connecting the ICD3, it reports:


  Target detected
  
  Device ID Revision = B4 B6


(For some reason, Microchip put the same revision ID code in revisions B4 and B6 for my device)

I don't think Microchip provides documentation to get the revision from the printed date-code. You could try squeezing it out of your local Microchip rep :)