New S3 mini Playstore Not Opening

Hey guys I just bought a S3 mini ,While buying the shopper gave me lesser price but said it wont carry warranty.
I purchased it. After Starting it worked properly.Installed apps from Playstore,But after few hrs even I can browse web but I cant open playstore using both the Playstore App and webpage.Webpage says certificate issues.I can Browse other websites.
If I reset Mobile I can acces playstore again but after sometime same problem