When the source published it, as the listing states it.

May be a date string ("YYYY-MM-DD HH:MM", RFC 3339, ...), a numeric string, Unix seconds, a JavaScript millisecond timestamp, or a Date. The dispatcher normalises numeric values and Date objects to a Unix-seconds string before the host receives them; date strings are parsed host-side. Declaring it here is what lets the host offer "published more than N hours ago" as a filter on any source carrying a date, rather than on the ones it was taught about individually. Omit it rather than guessing: a wrong value silently holds a work back, or releases one early.