/* gmtoff_tm.c - Get offset from GMT by calling gmtime and subtracting
 *
 * Copyright (c) 1994-2008 Carnegie Mellon University.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The name "Carnegie Mellon University" must not be used to
 *    endorse or promote products derived from this software without
 *    prior written permission. For permission or any legal
 *    details, please contact
 *      Carnegie Mellon University
 *      Center for Technology Transfer and Enterprise Creation
 *      4615 Forbes Avenue
 *      Suite 302
 *      Pittsburgh, PA  15213
 *      (412) 268-7393, fax: (412) 268-7395
 *      innovation@andrew.cmu.edu
 *
 * 4. Redistributions of any form whatsoever must retain the following
 *    acknowledgment:
 *    "This product includes software developed by Computing Services
 *     at Carnegie Mellon University (http://www.cmu.edu/computing/)."
 *
 * CARNEGIE MELLON UNIVERSITY DISCLAIMS ALL WARRANTIES WITH REGARD TO
 * THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS, IN NO EVENT SHALL CARNEGIE MELLON UNIVERSITY BE LIABLE
 * FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN
 * AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
 * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <config.h>

/*
 * Returns the GMT offset of the struct tm 'tm', obtained from 'time'.
 */
EXPORTED int gmtoff_of(struct tm *tm, time_t time)
{
    struct tm local, gmt;
    struct tm *gtm;
    long offset;

    local = *tm;
    gtm = gmtime(&time);
    gmt = *gtm;

    /* Assume we are never more than 24 hours away. */
    offset = local.tm_yday - gmt.tm_yday;
    if (offset > 1) {
	offset = -24;
    } else if (offset < -1) {
	offset = 24;
    } else {
	offset *= 24;
    }

    /* Scale in the hours and minutes; ignore seconds. */
    offset += local.tm_hour - gmt.tm_hour;
    offset *= 60;
    offset += local.tm_min - gmt.tm_min;

    /* Restore the data in the struct 'tm' points to */
    *tm = local;
    return offset * 60;
}