>> help timeidx
TIMEIDX returns numeric index given time string.
D = TIMEIDX(T) returns index value in relative days which is suitable for
DATENUM math. To work in hours multiply by 24, for seconds multiply
by 24*3600, etc. T can be a time string (DD:HH:MM:SS) where DD, HH
and MM are incrementally optional and SS can include a decimal
fraction. For multiple time values T must be a cell array of strings.
The result is the same size/shape but type DOUBLE.

D = TIMEIDX(T1,T2) returns a range of values. T1 and T2 are individual
time strings (as above) and the result is a 1x2 DOUBLE array.

N = TIMEIDX(...,FS) instead returns index values in sample units relative
to the beginning of a record where FS is samples/second. Sample
values are rounded to integer values. Time 0 corresponds to sample 1.

Example 1: operation times out after 5 minutes
t2 = now+timeidx('5:00')*24*3600;
while ~done && now<t2
%wait for something else to happen
end
if ~done
error('Operation failed to complete in 5 minutes')
end

Example 2: specify portion of WAV file to read
range = timeidx('2.5','1:02.5',Fs); %1 minute after 2.5 second delay
x = wavread('myfile.wav',range);

See also DATENUM, DATESTR