Query not producing expected result

I need help figuring out why a query is not returning the records I expect
it to. I'm searching on a DateTime column (timestamp without time zone -
not nullable). The query includes:

AND event.Primaryresp_fkey = 5000011 AND event.DateTime <= 'May-1-2019'
AND event.EventDone < 1

This does not return a record that has a DateTime value "May 1, 2019 9:52
AM". If I modify the query to:

AND event.Primaryresp_fkey = 5000011 AND event.DateTime <= 'May-2-2019'
AND event.EventDone < 1

it does return the record in question. I assume I am not doing the data
comparison correctly, but don't know why.

Chuck Martin
Avondale Software