I am design a website, witch could allow custom to give rss from search result. To avoid a large query, the best way may be create static .xml files. Then according the xml create date, do an automatic update per 24 hours.

I think the better way is use php to make a judge, if time()-filectime>24*60*60, do a .xml update, else just read the old .xml file. But, how to do this judge? custom will read the .xml directly, and xml can not make a `time()-filectime`

Or any better way to suggest me? Many thanks.