Full Moon in Iyar 5779

Shalom,

I was under the impression that a full moon occurs on the 15th day of every Jewish month. This year (5779), the 15th day of Iyar fell on May 20, 2019 of the civil calendar, two full days after the occurrence of the full moon, which was May 18, 2019 2:12pm Vancouver BC time. Could you please explain this discrepancy to me? Thank you very much.