The split function was removed in PHP 7. Can be replaced by preg_split or explode depending on the circumstances.
I'm guessing this is the plugin you're using? In which case you'll need to change line 9 of gregorian2jalali-overrides.php from this:
@list($g_y, $g_m, $g_d)=split('[,]',date("Y,m,d",$timestamp));
@list($g_y, $g_m, $g_d) = explode(',',date("Y,m,d",$timestamp));
Hope that helps. If you're able to report the issue (and fix) to the developer, please do.