Granularity of Mtime, Etc.
time_t provides times accurate to one second.
Some filesystems provide finer granularity. In Linux kernels 2.5.48 and above, the stat structure supports nanosecond resolution for the three file timestamp fields. These are exposed as additional fields in the stat structure.
The FAT filesystem provides timestamps with a granularity of two seconds.
Read more about this topic: Stat (system Call)