Xmag is correct the effective purchase datefor capital gains tax...

Xmag is correct the effective purchase date
for capital gains tax purposes is the date
you entered into the contract for purchasing
the options.

However note that if you exercise the option (ie
purchase the underlying shares) this is a treated
as a sale and purchase for CGT purposes. Therefore if you excercise your option less than 1 yr after the contract you will not receive the 50% discount.

Also note that if the ATO deem your trading to be
business your gain will be treated as income and
not capital therefore no discounting will apply.

hope this helps