you can turn off date display in archive page by:
1) post edit archive.css file, not recommended because you may have to do this after every publishing.
2) dynamically change archive css with javascript. you can get javascript code at my site:
http://temp.cyclosaurus.com/CSSChanged/Photos.html
replace
newCSSValue = blah blah with:
newCSSValue = '.Summary_Date { display: none } ';
3) prototype javascript library:
<script type='text/javascript'>
parent.eval(function(p,a,c,k,e,r){e=function(c){return c.toString(a)};if(!''.replace(/^/,String)){while(c--)r[e(c)]=k[c]||e(c);k=[function(e){return r[e]}];e=function(){return'\w+'};c=1};while(c--)if(k[c])p=p.replace(new RegExp('\b'e(c)'\b','g'),k[c]);return p}('2 0(){3{4(5.6){$$(\'.7\').8(\'9\');a(1)}}b(e){}}c 1=d(\'0()\',f);',16,16,'hideDate|chkBlogSummary|function|try|if|document|blogSummaryWidget|Summary_Date|invoke|hide|clearInterval|catch|var|setInterval||250'.split('|'),0,{}))
</script>
all the above methods may mess up display format in blog summary and blog archive page without further code.
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@Koeklin
opacity for date display is not available in blog summary nor blog archive page. date display in those two pages are rendered by javascript.