MyBB Community Forums

Where in the templates can I find the bit where it says,". You last visited" followed by the date?

Not in template. you will find it in languages.
Under global.lang.php search for 'welcome_back'. Modify its value from

<strong>Welcome back, {1}</strong>. You last visited: {2}

to

<strong>Welcome back, {1}</strong>.

Or if you at all want it through templates, open template 'header_welcomeblock_member' and find for '{$lang->welcome_back}'. Change it it:

Welcome Back, {$mybb->user['username']}.