The compiler receives warnings when using the strptime (C) function

Input man strptimemeans that this function was to declare _XOPEN_SOURCE and include the time.h header. I have done it. But, when I try to compile my code, I get:

./check.c: 56: warning: implicit declaration of function 'strptime

Take a look at my code:

int lockExpired(const char *date, const char *format, time_t current) {
        struct tm *tmp = malloc(sizeof(struct tm *));
        time_t lt;
        int et;

        strptime(date, format, tmp);
        lt = mktime(tmp);
        et = difftime(current, lt);

        if (et < 3600)
                return -et;

        return 1;
}

Also function declaration:

char *strptime(const char *s, const char *format, struct tm *tm);

Can someone tell me where my problem comes from?