The date is not certain.  We know they were in port on September 15 and this is the "second day out", but we don't know precisely on what day they sailed.  This uncertainty is perpetuated in the next few entries.

(Arabella here dates the Spanish raid as "three years ago" when it was actually just over two years ago.  Could this mistake be due to terror at her captive state, or could she have been momentarily dazzled by Lord Julian's effulgence?  Neither hypothesis seems likely.)