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Date: Sat, 23 Mar 2024 17:36:00 -0700
From: Eric Pruitt <>
Subject: Re: Broken mktime calculations when crossing DST boundary

On Sat, Mar 23, 2024 at 08:40:50PM +0000, Alexander Weps wrote:
> Is there a way, how to reliable get beginning of day etc. without tm_isdst = -1.

I needed to do something similar in a program that involved sunset and
sunrise calculations. Depending on what you're doing and the information
you have, maybe the approach I used could work:

    static time_t round_down_to_midnight(time_t when)
        char buf[20];
        struct tm *in;
        struct tm out;

        when -= when % 60;
        in = localtime(&when);

        if (in->tm_hour == 0 && in->tm_min == 0) {
            return when;

        sprintf(buf, "%d-%02d-%02d 00:00:00", in->tm_year + 1900, in->tm_mon + 1,
        strptime(buf, "%Y-%m-%d %H:%M:%S", &out);
        return mktime(&out);

The performance of this is not great, though.


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