After selecting a Month or Day, the change to the Julian Date is immediate.

When selecting a different Month, or entering a different Year, the displayed day will remain
constant, unless the number of days in the selected Year/Month combination is less than the displayed day.
In this case the displayed day will be set to the last day of the selected Month.

After entering a new Year, click anywhere in the window (outside the Year field) to see the new Julian Date.

Pressing the enter key, after changing the Year, is the same as pressing reset;
(the display returns to the current date).