If your date string always ends in the following 13 characters: " 00:00:00 GMT", then this should work:
newDate = Left (dateString, Len (dateString) - 13)
Although, if it may also be of the form " 0:00:00 GMT" then you might want to use:
if Mid (dateString, Len (dateString) - 11, 1) = " " then
newDate = Left (dateString, Len (dateString) - 12)
else
newDate = Left (dateString, Len (dateString) - 13)
endif
I haven't tested it out since I don't have my Roku with me, but that should give you somewhere to start. I believe you could also parse your date using an roRegex object (see the Component Reference Manual, section 5.5).