@carletongardener
you could use a couple of checkboxes to set the dates (tomato, cucumber, ...) to be computed. the checkboxes' value could be the number of days to subtract/add to the current date. this way, the function triggered by the button needs to fetch all the checked checkboxes using their passed value to compute the date.
for a better styling of the page have a look at CSS, especially the properties
padding and
margin should be good to start.
finally, I think you'd better use HTML than XHTML to start with
- true XHTML (with the recommended mime type "application/xhtml+xml") is not supported by Internet Explorer
- XHTML served as "text/html" is treated as HTML
- XHTML must be well-formed, otherwise browsers give you an error (the so-called Yellow Screen Of Death (YSOD))
(empty elements (like <img>, <br>, <hr>) are written <
tagname/> (<img/>, <br/>, <hr/>), & must always be escaped)
regards