Please see the image below.
From the above figure, there are different time information like Start_time, Stop_time, creation time, and History. I want to know the exact time when the Sentinel 3A recorded the data. Which filed shall I consider for it?
The first line was recorded at start_time and the last line was recoded at stop_time. In between you can interpolate.
This is what SNAP shows in the Pixel Infow view.
I think for more accurate time information you need interprete the data given in time_in on your own.
See also the documentation Sentinel-3-SLSTR-Product-Data-Format-Specification-Level-1 at page 33.