Prophetic messages often include exact dates to place their events in a clear historical context. This particular vision is a later prophecy focused on the nation of Egypt [ביאור שטיינזלץ]. It takes place during an eleventh year, which marks a tragic historical milestone: the year the Temple was destroyed.
The starting point for counting these eleven years is a matter of discussion. The timeline is either calculated from the exile of King Jehoiachin and the reign of King Zedekiah [מצודת דוד], or it is based directly on the exile of Zedekiah himself [רד״ק]. The exact time of year is pinpointed to the very beginning of the calendar, specifically the first month, known as Nissan [רד״ק, ביאור שטיינזלץ].