where = empty field get a wrong result

With the where clause, getResources queries modResource standard object fields. I suspect that your datestart and dateend are TVs, not resource fields. What you need is the tvFilters property or a more complex query using a custom snippet.

With &tvFilters you could do this (untested):

&tvFilters=`datestart>=[[!today]]%,dateend>=[[!today]]%`

Also, I'd advise converting your dates to unix timestamps when comparing against another date. That way you're dealing with numbers instead of strings.

How about a different approach? Since it sounds like you are using getResources can you use the publish and unpublish dates?

I am using this...

[[!getPage@Events?
     &elementClass=`modSnippet`
     &element=`getResources`
     &includeTVs=`1`
     &tvPrefix =``
     &tpl=`tpl.Events`
     &showHidden=`1`
     &sortbyTV=`startTime`
     &sortdirTV=`ASC`
     &sortbyTVType=`datetime`
]]

<div class="text-center">
     <ul class="pagination">[[!+page.nav]]</ul>
</div>

So you're using MIGX, not getResources? It would be useful if you posted your full getPage call. All the answers, mine included, have assumed you were using getResources in your getPage call. FYI Articles uses getResources to list resources and I'm not sure you'll be well served with Articles.

[[!getPage?
					&element=`getImageList`
					&tpl=`agenda.aside`
					&docid=`16`
					&limit=`5`
					&tvname=`agenda`
					&sort=`[{"sortby":"datestart","sortdir":"ASC"}]`
					&where=`{"datestart:>=":"[[!today]]"},{"OR:dateend:>=":"[[!today]]"}`
				]]

I would use a custom-table and a MIGXdb - TV or create a MIGXdb - CMP