/usr/bin/datemail is in qmail 1.06-4.
This file is owned by root:qmail, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 | #!/bin/sh
# WARNING: This file was auto-generated. Do not edit!
exec /var/lib/qmail/bin/predate /var/lib/qmail/bin/sendmail ${1+"$@"}
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