Discount Tax Rate

Question for any tax expert out there.
Where the exercise of options is concerned, is the date of when the options were purchased used as the start of the 12 months required to recieve the discount rate? Or the date that appears on your Chess statement once exercised?
In HDR's case the 13th of September is the date that appears on the Chess Statement of exercised options.
Any help appreciated.

Regards,
xmagx