javashit datetime localization help

how do i make
<span class="datetime">1234567890</span>
become <span class="datetime">2009-02-13 18:31</span>
(eg for someone in eastern time; shows in user's local timezone)

pretend i have a lot of <span class="datetime"> scattered about and i want it to either be rendered in local time or in relative "time until" / "time since" units (eg `3 days, 5 hours ago`...)

thanks

What programming language is this?