SLT

This stands for Second Life Time, which is the time as displayed in the upper right corner of the Second Life client. From the Second Life Knowledge Base article:

SLT stands for Second Life Time, which the in-world clock displays near the top-right corner of the screen. Second Life Time is the same as Pacific Time (PDT or PST depending), because San Francisco is where the main offices of Linden Lab, makers of Second Life, are situated.