Hi John,
You're not getting an extraneous "21" - you're actually getting "121". May 1st is day 121 of the year (yes, I had to look that up ).
Please use the Java documentation for SimpleDateFormat as a guide:
https://docs.oracle.com/javase/8/docs/api/
Capital "D" is for day of the year. Lowercase "d" is for day of the month. The uppercase "Y" might cause problems, too.
Try: "yyyy-MMM-dd" for "2018-May-01" or "yyyy-MM-dd" for "2018-05-01". There are other variations, too.
--Mike