Analysis of information sources in references of the Wikipedia article "Laguna Copperplate Inscription" in English language version.
It involved the reconstruction of an ancient calendar used in the Philippines over a millennium ago using clues provided by Indian and Indonesian calendars from which it was derived. Let me present the answer right at the start. Then I will show how this equivalent date was derived. The Julian equivalent is: Monday, 21 April 900 A.D
A copper plate containing an Old Malay inscription [...] deciphered by the Dutch Ethnographer [sic] Antoon Postma, carries a clear date of Saka 822, a Sanskritized reckoning equivalent to A.D. 900.