I am trying to interpolate acceleration values but am getting an error.. can somebody guide me to rectify it ?

if true
  % code
end  
num=xlsread('HASC24274-acc.csv'); 
timestamps = num(:,1);
signals = num(:,2:end);
first_time = timestamps(1);
last_time = timestamps(end);
Fs = 50;
period = datenum(1/Fs);
%period = datenum([0 0 0 0 0 1/Fs]);
probe_times = first_time : period : last_time;
resampled_data = interp1(timestamps, signals, probe_times);

 end

if true
  % code
endif true
  % code
Error using griddedInterpolant
The grid vectors are not strictly monotonic increasing.
Error in interp1 (line 158)
      F = griddedInterpolant(Xext,V,method); 
end