/* gmtoff_tm.c - Get offset from GMT by calling gmtime and subtracting
 $Id: gmtoff_gmtime.c,v 1.9 2003/02/13 20:15:40 rjs3 Exp $
 
 * Copyright (c) 1998-2003 Carnegie Mellon University.  All rights reserved.
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 * modification, are permitted provided that the following conditions
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 * 1. Redistributions of source code must retain the above copyright
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 * 2. Redistributions in binary form must reproduce the above copyright
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 *    the documentation and/or other materials provided with the
 *    distribution.
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 * 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 other legal
 *    details, please contact  
 *      Office of Technology Transfer
 *      Carnegie Mellon University
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 *    acknowledgment:
 *    "This product includes software developed by Computing Services
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 * 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
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#include <config.h>
#include <time.h>

/*
 * Returns the GMT offset of the struct tm 'tm', obtained from 'time'.
 */
int gmtoff_of(tm, time)
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;
}
