CALENDAR
The date may be set either manually or by clicking on a increment button. The month will increment through December to January and the other way, and also change the year to the correct value.
If either a new month is selected or a new year is typed in the box, one must click the Get Date button. The Today button returns the Calendars to the original display. Years below 1583 are not allowed due to the calendar change that took place in 1582.
There are comments in the code to change the display so that the current month is last or first, and to set the number displayed rather than asking.

DIFFERENCE
The two dates may be set either manually or by clicking on a calendar above.
Clicking on a calendar above will set the To date to that date and move the old To date to the From date.
Clicking the Now button on either date will set that date to today
Clicking Get Diff will give the number of calendar days between the two dates above, either positive or negative

JULIAN
Returns the Julian date for the To Date at 12 noon UTC.  This is the time the Julian day turns rather than at midnight.

ADJUST
The days, + or -, in the text area are added to the To date above and the result is shown in the display box below. Time is set for 4 am for the To Date; time shown indicates difference due to DST.