folding archives

Hi,
I am trying to install the folding archives plug in, but when I put in on the sidebar the error comes up: “Archives
Fatal error: Call to undefined function date_timestamp_get() in /home3/[myname]/public_html/wp-content/plugins/folding-archives/index.php on line 35″

Does anyone know how to fix this or whether there’s a newer version of the plug in that will work?
I don’t know what version of WP I am using.

Thanks for your help!!