Contemporaneous, an ensemble of some two dozen musicians, started out at Bard College as the brainchild of a pair of undergrads. Now, more than a decade later, the ensemble is based in New York City and continues to thrive professionally. It will present its largest production to date on Sept. 18. Billed as The Day of Imagination, the program at the Irondale Center in Brooklyn will feature three sets over a full day, four world premieres, six hours of music, and 50 artists.