Sept 2015?

No.

May 2016.

Sept 2015 is wishful thinking and the projection of impatient minds.

Or these impatient minds could be right.

I doubt it.

Here's one analysis why it is unlikely to be Sept 2015.

I seem to recall that during Budget 2015, it was noted that this year's budget will incur a deficit of about $6b. And there were some programmes that have been extended to 2016 and beyond which means that the 2016 budget will also incur a deficit. But this is fine as the govt has accumulated surpluses over the last few years, and can offset the deficit with those surpluses.
BUT, if the election is before April 2016, parliament will be dissolved and there would be a new government (new parliament) in FY2016, and there would be NO surpluses to offset the budget deficit. 

If there is a planned deficit for the FY2016 budget, then the GE cannot be before April 2016.

BUT, I could be wrong.

[Edit/Afternote (6 Aug) : Then Minister for Transport announced fare decrease of 1.9% at the end of the year, and the opening of 12 stations on the Downtown Line 2 on Dec 27. 

And it is seemingly more certain that Sept 2015 is not going to be elections.

Still thinks it will be May 2016.]