O MySQL tem suporte bastante misterioso para fusos horários. Talvez isso faça o que você quer:
select convert_tz(str_to_date(left(val, 15), '%Y%m%dT%H%i%s'), '+00:00', insert(right(val, 5), 4, 0, ':'))
from (select '20080915T155300+0500' as val) x
select convert_tz(str_to_date(left(val, 15), '%Y%m%dT%H%i%s'), '+00:00', insert(right(val, 5), 4, 0, ':'))
from (select '20080915T155300+0500' as val) x
