strptime() only touches the fields specified in the format string,

strptime() only touches the fields specified in the format string,
meaning there could be garbage left in the other fields. Somehow this
only caused issues in mktime() when /etc/localtime is set to GMT.
Initialize tm to 0.

While here fix a type-O in the format string and make the invalid
strlen for LAST-UPDATED message more consistent with the other 2 error
messages.

Found by and OK sthen@