I got the error when I first tried to build my sitemap as well as get the same error when I try to rebuild the sitemap.
Warning: mktime() expects parameter 1 to be long, string given in /home/public_html/wp-content/plugins/google-sitemap-generator/sitemap-core.php on line 2438
The main problem is that the plugin gets stuck in a loop. When that error comes up it's as if the plugin keeps trying and trying and trying. I have to click on some link in WP so that it will end it. I have disabled the plugin for now just in case when I add content it tries to rebuild the sitemap. I wouldn't want an infinite loop to end up starting without me knowing it. I'm sure my host would have a cow.