Depois de remover um espaço entre date
e +
em seu código fornecido, vejo arquivos girados sendo criados:
[nevin-mac-mini:~] root# tcpdump -i en0 -nvv -w /tmp/encrypted_date+%y_%m_%d_%H_%M_%S.pcap -G 10
tcpdump: listening on en0, link-type EN10MB (Ethernet), capture size 65535 bytes
^C4875 packets captured
4876 packets received by filter
0 packets dropped by kernel
[nevin-mac-mini:~] root#
[nevin-mac-mini:~] root# ls /tmp/encr*
/tmp/encrypted_date+15_09_13_18_47_49.pcap
/tmp/encrypted_date+15_09_13_18_48_01.pcap
/tmp/encrypted_date+15_09_13_18_48_11.pcap
/tmp/encrypted_date+15_09_13_18_48_22.pcap
/tmp/encrypted_date+15_09_13_18_48_34.pcap
/tmp/encrypted_date+15_09_13_18_48_46.pcap
/tmp/encrypted_date+15_09_13_18_48_56.pcap
[nevin-mac-mini:~] root#
Talvez ter o -G
vindo após o -w
ser significativo ...?