GMT time conversion to Unix epoch

I am trying to convert the GMT time into unix epoch (starting from 1970), looks like there is a small bug
in code , i could not get and exhausted.

I see following difference

Non leap years
Expected output: Jan 20 19:00:01 2019 GMT = 1548010801
Actual output: 1548097201 (which is Jan 21 19:00:01 2019 GMT, 1 day difference)

Leap Year
Expected output: Dec 27 14:52:30 2020 GMT = 1609080750

Actual output: 1609253550 (which is Dec 29 14:52:30 2020 GMT, 2 days difference)

I really appreciate any help in finding the problem.

`
#include <stdio.h>
#include <string.h>
#include <time.h>

#define BASE_YEAR 1970

void print_time_readable_format(struct tm tm);
int convert_gmt_date_time_to_tm_format(char* gmt_time_fmt);
int check_year_is_leap_or_normal(int year);
int get_number_of_leap_years_from_base_year(int start_year, int end_year);
int convert_gmt_to_epoch(struct tm tm);

int main()
{
    int epoch = 0;
    //char gmt_time_fmt[] = "Jan 20 19:00:01 2019 GMT";
    //char gmt_time_fmt[] = "Dec 27 14:52:30 2020 GMT";
    char gmt_time_fmt[] = "Jan 01 00:00:00 1970 GMT";
    epoch = convert_gmt_date_time_to_tm_format(gmt_time_fmt);
    printf("time in GMT = %s and epoch is %d\n", gmt_time_fmt, epoch);

    return 0;
}

int convert_gmt_date_time_to_tm_format(char* gmt_time_fmt)
{
    struct tm tm;
    char tm_time_fmt[255];

    //set tm struture to 0
    memset(&tm, 0, sizeof(struct tm));
    // convert gmt_time_fmt to format required by 'tm' structure
    strptime(gmt_time_fmt, "%B %d %H:%M:%S %Y GMT", &tm);

    strftime(tm_time_fmt, sizeof(tm_time_fmt), "%s", &tm);
    printf("tm_time_fmt = %s\n", tm_time_fmt);

    print_time_readable_format(tm);

    return convert_gmt_to_epoch(tm);

    return 0;
}

int convert_gmt_to_epoch(struct tm tm)
{
    int days_by_month [2][12] = {
        /* normal years */
        { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334},
        /* leap years */
        { 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335}
    };

    int current_year = tm.tm_year+1900;

    printf("current_year =%d\n", current_year);

    int total_years_passed = current_year - BASE_YEAR;

    printf("total_years_passed =%d\n", total_years_passed);

    int nleap_years_passed = get_number_of_leap_years_from_base_year(BASE_YEAR, current_year);

    int normal_years = total_years_passed - nleap_years_passed;

    printf("normal_years =%d\n", normal_years);

    int total_days_passed = (normal_years*365 + nleap_years_passed*366 );

    printf("total_days_passed =%d\n", total_days_passed);

    total_days_passed += days_by_month[check_year_is_leap_or_normal(current_year)][tm.tm_mon];

    printf("total_days_passed after adding month =%d\n", total_days_passed);

    total_days_passed += tm.tm_mday;

    printf("total_days_passed after adding day =%d\n", total_days_passed);

    total_days_passed  = total_days_passed*24 + tm.tm_hour;
    total_days_passed  = total_days_passed*60 + tm.tm_min;
    total_days_passed  = total_days_passed*60 + tm.tm_sec;

    printf("total_days_passed final =%d\n", total_days_passed);

    return total_days_passed;

}

int get_number_of_leap_years_from_base_year(int start_year, int end_year)
{
    int leap_year_count = 0;
    int year = start_year;

    while( year <= end_year)
    {
        if(check_year_is_leap_or_normal(year))
            leap_year_count++;
        year++;
    }

    printf("leap_year_count = %d\n", leap_year_count);

    return leap_year_count;
}

int check_year_is_leap_or_normal(int year)
{
    if( ( year%4 == 0 ) && ( ( year%400 == 0 ) || ( year%100 != 0)))
         return 1;
    else
        return 0;
}

void print_time_readable_format(struct tm tm)
{
    printf("tm.tm_year = %d ", tm.tm_year);
    printf("tm.tm_mon = %d ", tm.tm_mon);
    printf("tm.tm_mday = %d ",tm.tm_mday);
    printf("tm.tm_hour = %d ", tm.tm_hour); 
    printf("tm.tm_min = %d ", tm.tm_min );
    printf("tm.tm_sec = %d\n", tm.tm_sec );
}
`