Implement valid_before() and valid_after().
Implement valid_before() and valid_after().

VirusTotal/yara,kallanreed/yara,hillu/yara,wxsBSD/yara,beikezcs/yara,rayyang2000/yara,kallanreed/yara,plusvic/yara,bmagistro/yara,bmagistro/yara,pombredanne/yara,binsrc/yara,msuvajac/yara,tanium/yara,wxsBSD/yara,digideskio/yara,digideskio/yara,0x0mar/yara,rayyang2000/yara,neatsun/yara,plusvic/yara,plusvic/yara,bmagistro/yara,plusvic/yara,0x0mar/yara,wxsBSD/yara,Security513/yara,pombredanne/yara,rednaga/yara,jpypi/yara,cblichmann/yara,cblichmann/yara,pombredanne/yara,beikezcs/yara,binsrc/yara,msuvajac/yara,jshufelt/yara,smaudet/yara,neatsun/yara,smaudet/yara,tdr130/yara,VirusTotal/yara,tanium/yara,foreni-packages/yara,VirusTotal/yara,foreni-packages/yara,jpypi/yara,VirusTotal/yara,jshufelt/yara,frbapolkosnik/yara,tdr130/yara,cblichmann/yara,mgoffin/yara,neatsun/yara,hillu/yara,pombredanne/yara,wxsBSD/yara,beikezcs/yara,hillu/yara,rednaga/yara,digideskio/yara,binsrc/yara,frbapolkosnik/yara,0x0mar/yara,plusvic/yara,tanium/yara,Security513/yara,jshufelt/yara,tdr130/yara,rednaga/yara,foreni-packages/yara,msuvajac/yara,Security513/yara,smaudet/yara,digideskio/yara,VirusTotal/yara,hillu/yara,frbapolkosnik/yara,frbapolkosnik/yara,mgoffin/yara,wxsBSD/yara,mgoffin/yara,0x0mar/yara,smaudet/yara,jpypi/yara,binsrc/yara,hillu/yara,rayyang2000/yara,mgoffin/yara,cblichmann/yara,MeteorAdminz/yara,cblichmann/yara,kallanreed/yara,pombredanne/yara,MeteorAdminz/yara,MeteorAdminz/yara,jshufelt/yara