What is the corresponding XTickLabel for the time (t) in this code?

time_axis = ['13:45'
'13:46'
'13:47'
'13:48'
'13:49'
'13:50'
'13:51'
'13:52'
'13:53'
'13:54'
'13:55'
'13:56'
'13:57'
'13:58'
'13:59'
'14:00'
'14:01'
'14:02'
'14:03'
'14:04'
'14:05'
'14:06'
'14:07'
'14:08'
'14:09'
'14:10'
'14:11'
'14:12'
'14:13'
'14:14'
'14:15']; 
ax.XTick = [0,240,480,720,960,1200,1440,1680,1920,2160,2400,2640,2880,3120,3360,3600,...
    3840,4080,4320,4560,4800,5040,5280,5520,5760,6000,6240,6480,6720,6960,7200]; 
ax.XTickLabel = time_axis; 
t = [2842.6053
3185.8029
3464.6509
3722.0492
4143.8962
4508.5437
4894.6410
5409.4374
5816.9846
6145.8823]; 

How can I find at which time_axis (or XTickLabel) is t (in the format hh:mm:ss)?

Many thanks!