minimal parsing of rfc-1123 date-time strings

trivial-rfc-1123

### TRIVIAL-RFC-1123

Parses and prints dates in [RFC-1123](https://tools.ietf.org/html/rfc1123) format.  Like this:
```
CL-USER> (ql:quickload :trivial-rfc-1123)
CL-USER> (t1123:parse-date "Fri, 16 Jul 2010 02:33:50 -0500") ;note 't1132:' abbrev.
3488254430
CL-USER> (t1123:as-rfc-1123 3488254430); default tz is 0, or GMT
"Fri, 16-Jul-2010 07:33:50 GMT"

CL-USER> (t1123:as-rfc-1123 3488254430 :timezone 5); 'cause lisp TZ is backwards
"Fri, 16-Jul-2010 02:33:50 -0500"

CL-USER> (t1123:as-rfc-1123 (get-universal-time) :timezone nil); use current tz!
...


```
## (parse-date string)

### Return: universal time for the string.

date-parse-error condition will be raised in case of problems.

## (as-rfc-1123 universal-time &key (stream nil) (timezone 0)

### Stream:
- nil return string
- t output to *standard-output*
- valid stream - output to stream.

### Timezone:
- Specify timezone for output.  Lisp timezone, please (it is backwards from the normal offset: for GMT-08:00, specify 8).  Only rational timezones are supported.  Nil means use current tz from (decode-universal-time) which is useful for current time.

### Return: string if stream is nil, nil otherwise.

## License

Parts are subject to "Edi Weitz" license, and the entire surrounding project, a similar BSD 3-clause license.  The lowdown: credit copyright ownership to Dr. Edward Weitz and Stacksmith, pack licenses and disclaimers with your code and distributables, and do not use the authors' names without permission for any unreasonable purpose.

## History

The code started out in drakma and was shoehorned to be a standalone library for applications requiring minimal date parsing and printing.

According to Drakma authors, the following formats will be parsed:

```
    Wed, 06-Feb-2008 21:01:38 GMT
    Wed, 06-Feb-08 21:01:38 GMT
    Tue Feb 13 08:00:00 2007 GMT
    Wednesday, 07-February-2027 08:55:23 GMT
    Wed, 07-02-2017 10:34:45 GMT
```
Note: Drakma is not clear on this, but if a timezone offset is specified, it is always considered to be from GMT.  It will accept "PST-02:00", but the result is same as "GMT-02:00", or plain "-02:00".

The following was added to the drakma code, based on empirical studies of NNTP dates:
```
    Fri, 07-02-2017 10:34:45 +0800   numeric timezone is ok - 4 digits HHMM
    Fri, 07-02-2017 10:34:45 +08:00  colon is ok HH:MM
    07-02-2017 10:34:45 GMT          omitting day is ok
	WEd, 06-Feb-08 21:01:38 GmT      no longer case sensitive
```

A limited output function was added.  For more flexible output (such as better timezone handling), see [local-time](https://www.common-lisp.net/project/local-time/) - which sadly does not parse RFC-1123, but outputs it.

References:
[RFC-1123](https://tools.ietf.org/html/rfc1123) 
[RFC-822 Section 5](https://tools.ietf.org/html/rfc822#section-5)


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