man rsync

I always wondered about that.

--modify-window

When comparing two timestamps rsync treats the timestamps as being equal if they are within the value of modify_window. This is normally zero, but you may find it useful to set this to a larger value in some situations. In particular, when transfer- ring to Windows FAT filesystems which cannot represent times with a 1 second resolution --modify-window=1 is useful.

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DIAGNOSTICS
BUGS

times are transferred as unix time_t values

When transferring to FAT filesystems rsync may re-sync unmodified files. See the comments on the --modify-window option.