Very cool find OptimumPX! The photo you reference is said to have been taken in 1927. In that photo, 15 Broad appears nearly complete, without any scaffolding or derricks, as my photo has. The trees in battery park also look to be barren, so definitely a winter photo.
The photo below is claimed to be from July of that same year, and shows 15 broad well under construction.
In my photo leaves are clearly present on the trees in the foreground so it can't be too late in the year. The Urban Politician may have just been trolling, but his 9/13 date might not actually be too far off.