Per Year

 @manual{bibtex-patashnik88,
    author = {Patashnik, Oren},
    month = {8}}}ing},
    url = {http://mirrors.ctan.org/biblio/bibtex/base/btxdoc.pdf},
    year = 1988
} 

 @manual{bibtexstyles-patashnik88,
    author = {Patashnik, Oren},
    month = {8}}} Styles},
    url = {http://mirrors.ctan.org/biblio/bibtex/base/btxhak.pdf},
    year = 1988
}