A simple parser converting RFC1123 date into an argument date util accepts.

rfc1123.c

/*
 * RFC1123 date parser.
 * Copyright (c) 2010 by Michal Nazarewicz (mina86/AT/mina86.com)
 * Distributed under the terms of Academic Free License 3.0
 */

#include <ctype.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>

static const char months[] = "janfebmaraprmayjunjulaugsepoctnovdec";
static const char tzs[] = "gmtutc";

static void     acceptWS(void);
static unsigned acceptStr(const char *strings);
static unsigned acceptInt(unsigned min, unsigned max);

static void ensure(int cond) {
	if (!cond) {
		exit(EXIT_FAILURE);
	}
}

static char *input;

int main(void) {
	unsigned day, mon, year, hour, min, sec;
	char line[128];

	ensure(fgets(line, sizeof line, stdin) != 0);
	input = line;
	acceptStr(0); ensure(*input++ == ',');
	day  = acceptInt(1, 31);
	mon  = acceptStr(months) + 1;
	year = acceptInt(2010, 2100);
	hour = acceptInt(0, 23); ensure(*input++ == ':');
	min  = acceptInt(0, 59); ensure(*input++ == ':');
	sec  = acceptInt(0, 59);
	acceptStr(tzs); acceptWS(); ensure(*input == 0);

	printf("%02u%02u%02u%02u%04u.%02u\n", mon, day, hour, min, year, sec);
	return EXIT_SUCCESS;
}

static void acceptWS(void) {
	while (isspace(*input)) {
		++input;
	}
}

static unsigned acceptStr(const char *strings) {
	char *f;

	acceptWS();

	for (f = input; 1; ++input) {
		if (isupper(*input)) {
			*input = tolower(*input);
		} else if (!islower(*input)) {
			break;
		}
	}

	ensure(input - f == 3);

	if (strings) {
		const char *s = strings;
		while (*s && (f[0] != s[0] || f[1] != s[1] || f[2] != s[2])) {
			s += 3;
		}

		ensure(*s != 0);
		return (unsigned)(s - strings) / 3u;
	}

	return 0;
}

static unsigned acceptInt(unsigned min, unsigned max) {
	unsigned long ret;

	errno = 0;
	ret = strtoul(input, &input, 10);
	ensure(!errno && ret >= min && ret <= max);
	return ret;
}