The date is unknown. We are rarely able to establish the exact date of events in Classical Greek history. Generally this is only possible if a source tells us the date on some Greek calendar (which almost never happens) or if they relate it to an astronomical event, like an eclipse, which we can usually date quite precisely. In this case the sources (mainly Thucydides) don't provide either information, so we are in the dark. Even the year 433 BC is only established by extrapolating from other parts of the text.