Bug 13406:
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14306
notes that if you use Edit > Time Shift to shift the time stamps of packets, you don't get to save the packets with the shifted time stamps.
Are there any reasons *not* to unconditionally have both Save/Save As and Export Specified Packets write out packets with the shifted time stamps? Other operations, such as "Export Packet Dissections", use the shifted time stamps, and if you *don't* want to save the packets with the shifted time stamps, you can always undo all the shifts.