function str = szeit(tend, tstart)
% write formated time to string

if (nargin > 1)
    dt = tend - tstart;
else
    dt = tend;
end

if (nargout > 0)
    str = sprintf('%i:%02i:%06.3f', fix(dt / 3600), ...
                  mod(fix(dt / 60), 60), mod(dt, 60));
else
    fprintf('%i:%02i:%06.3f\n', fix(dt / 3600), ...
            mod(fix(dt / 60), 60), mod(dt, 60));
end

return