dp man page
dp — parse dates RFC 822-style
/usr/libexec/nmh/dp [-form formatfile] [-format string] [-width columns] [-version] [-help] dates ...
Dp is a program that parses dates according to the ARPA Internet standard. It also understands many non-standard formats, such as those produced by TOPS-20 sites and some UNIX sites using ctime(3). It is useful for seeing how nmh will interpret a date.
The dp program treats each argument as a single date, and prints the date out in the official RFC 822 format. Hence, it is usually best to enclose each argument in quotes for the shell.
To override the output format used by dp, the -format string or -format file switches are used. This permits individual fields of the address to be extracted with ease. The string is simply a format string and the file is simply a format file. See mh-format(5) for the details.
Here is the default format string used by dp:
which says that if an error was detected, print the error, a `:', and the date in error. Otherwise, output the RFC 822-proper format of the date.
The user's profile.
Standard for the Format of ARPA Internet Text Messages (RFC 822)
As described above.
The width of the terminal.
ap(8), fmttest(1), mhl(1), nmh(7).