An inscription on a figure of a yaksha from Mathura in the reign of Kumaragupta has been dated to 432 CE, and a pedestal (with no king's name on it, but presumably from Kumaragupta's reign) has been dated to 442 CE [REF].
The Bilsad inscription is the oldest record of his reign and it dates to Gupta year 96, which corresponds to 415 CE.