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When a spider crawls your site it will update the database with the content found, which means that any modifications will be updated in the database.
The spiders will not crawl your site more frequently just because specify when the last modifications was made.
If your site is search engine friendly and it has got inbound links from other sites, which are already indexed, your site will be crawled and any modifications will be updated in the database.
You just have to be patient