Warranty issue on Desktop and Monitor

Hi, I just bought "Dell Vostro3900 Mini Tower (Later 2013)" with "Dell E2015Hv Monitor" separately  yesterday (22 March 2015). They said "both" have 3-years warranty which will be start on purchased date and can be check by using "Service tag" on dell website.

After I've checked, it seem like my desktop warranty is already started on "December 11, 2014"  and there is no "Service tag" for the monitor.

How should I proceed? 

Thanks.