you probably have to provide place holders in the fixed html, then replace those with your value, using "sed". e.g. test.html:
<html>
....
<div>The date is now %%DATE%%</div>
</html>
Then your script would replace %%DATE%%:
sed "s/%%DATE%%/$(date)/" test.html
Similarly you can replace environment varables:
export MYDATE="2013-12-27"
sed "s/%%DATE%%/$MYDATE/" test.html