longernewer Switch
The longernewer switch lets you transfer
only files that don't already exist on the destination system, files that
are longer, or files that are newer (have a more recent date and time).
The longernewer switch can be used with these file transfer
commands:
ztransmit and zreceive
You may not use the longernewer
switch at the same time as the append, different, hash,
newer, or nodelete switches.