How to convert xsl / xslx to csv on the server for uploading to the website database using cron?

Question

Parsing

There is a site to which I feed Excel files of the big sizes (millions of lines). Parsing this phpExcel thing is terribly long, too expensive for the server. I decided to convert the files to CSV and shove them into the database by cron. But I did not find any standard solution, because in server matters "Alyoshenka", no offense to a good name.
Paid ones are also beyond my power, the site is not commercial and does not bring any chips, except for its own self-satisfaction, so the question is for the sake of experiment
ps: I can’t force people to save immediately to csv