% Suppose you have a cellstring with dates in your third format:
s = {'2011-07-25T16:00:12-08:00'
'2011-07-25T16:00:18+07:00'};
% Separate the date from the timezone catching the + or - sign
[s,op] = regexp(s,'(?<=\d{2}:\d{2})-|+','split','match');
s = cat(1,s{:});
% Index the minus sign
idx = strcmp('-',cat(1,op{:}));
% To keep only the fraction of the day we offset by today date
t = fix(now);
% Dates with time zone conversion
out1 = datenum(s( idx,1),'yyyy-mm-ddTHH:MM:SS') - datenum(s( idx,2),'HH:MM') + t;
out2 = datenum(s(~idx,1),'yyyy-mm-ddTHH:MM:SS') + datenum(s(~idx,2),'HH:MM') - t;
