A very small library of north american centric date/time utilities

micro_time.h

#define ATLANTIC_TZ -5
#define EASTERN_TZ -5
#define CENTRAL_TZ -6
#define MOUNTAIN_TZ -7
#define PACIFIC_TZ -8
#define ALASKA_TZ -9
#define HAWAII_TZ -11

struct DateTime {
    int year;
    int month;
    int day;
    int hour;
    int hour12;
    int min;
    int sec;
    bool pm;
};

bool is_leap(int year) {
    return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
}

int day_of_week(int year, int month, int day) {
    if (month < 3) {
        month += 12;
        year -= 1;
    }

    int y = year % 100;
    int c = year / 100;

    int day_of_week_val = (day + (13 * (month + 1)) / 5 + y + y / 4 + c / 4 + 5 * c) % 7;

    return (day_of_week_val + 6) % 7;
}

int nth_weekday_date(int year, int month, int n, int weekday) {
    int first_day_weekday = day_of_week(year, month, 1);
    
    int diff = weekday - first_day_weekday;
    if (diff < 0) {
        diff += 7;
    }

    return 1 + diff + (n - 1) * 7;
}

bool is_daylight_savings(int year, int month, int day) {
    if (month < 3 || month > 11) {
        return false;
    }

    if (month > 3 && month < 11) {
        return true;
    }
    
    if (month == 3) {
        int dst_start_day = nth_weekday_date(year, 3, 2, 0);
        return day >= dst_start_day;
    }

    if (month == 11) {
        int dst_end_day = nth_weekday_date(year, 11, 1, 0);
        return day < dst_end_day;
    }

    return false;
}

void date_from_epoch(int64_t epoch_seconds, int& year, int& month, int& day) {
    const int64_t SECONDS_IN_DAY = 86400;
    int64_t total_days = epoch_seconds / SECONDS_IN_DAY;

    year = 1970;
    int64_t days_in_year;
    while (true) {
        days_in_year = is_leap(year) ? 366 : 365;
        if (total_days < days_in_year) {
            break;
        }
        total_days -= days_in_year;
        year++;
    }

    int days_in_month[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    if (is_leap(year)) {
        days_in_month[1] = 29; // Adjust for February in a leap year
    }

    month = 1;
    for (; month <= 12; ++month) {
        if (total_days < days_in_month[month - 1]) {
            break;
        }
        total_days -= days_in_month[month - 1];
    }

    day = static_cast<int>(total_days) + 1;
}

void datetime_from_epoch(int64_t epoch_seconds, DateTime& dt) {
    const int64_t SECONDS_IN_DAY = 86400;
    int64_t total_days = epoch_seconds / SECONDS_IN_DAY;
    unsigned long secs = epoch_seconds % SECONDS_IN_DAY;

    int year = 1970;
    int64_t days_in_year;
    while (true) {
        days_in_year = is_leap(year) ? 366 : 365;
        if (total_days < days_in_year) {
            break;
        }
        total_days -= days_in_year;
        year++;
    }

    int days_in_month[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    if (is_leap(year)) {
        days_in_month[1] = 29; // Adjust for February in a leap year
    }

    int month = 1;
    for (; month <= 12; ++month) {
        if (total_days < days_in_month[month - 1]) {
            break;
        }
        total_days -= days_in_month[month - 1];
    }

    dt.year = year;
    dt.month = month;
    dt.day = static_cast<int>(total_days) + 1;
    dt.hour = secs / (60*60);
    dt.min = secs % (60*60) / 60; 
    dt.sec = secs % 60; 
    dt.pm = dt.hour>=12;
    dt.hour12 = dt.hour;
    if (dt.hour==0) {
        dt.hour12 = 12;
    } else if (dt.hour>12) {
        dt.hour12 -= 12;
    }
}

int tz_offset(unsigned long gmtEpoch, int tz = MOUNTAIN_TZ, bool dst = true) {
  int year, month, day, offset = tz*60*60;
  date_from_epoch(gmtEpoch + offset, year, month, day);
  if (dst && is_daylight_savings(year, month, day)) {
    offset += 60*60;
  }
  return offset;
}

String formatted_date_time(unsigned long epoch, bool date = true, bool time = true, bool time12 = true, bool sec = false) {
    char buf[30];
    int count = 0;
    DateTime dt;
    buf[count] = 0;
    datetime_from_epoch(epoch, dt);
    if (date) {
        count += snprintf(buf+count, sizeof(buf) - count, "%4d/%02d/%02d", dt.year, dt.month, dt.day);
    }
    if (date && time) {
        buf[count++] = ' ';
        buf[count] = 0;
    }
    if (time) {
        count += snprintf(buf+count, sizeof(buf) - count, "%d:%02d", time12 ? dt.hour12 : dt.hour, dt.min);
    }
    if (time && sec) {
        count += snprintf(buf+count, sizeof(buf) - count, ":%02d", dt.sec);
    }
    if (time && time12) {
        count += snprintf(buf+count, sizeof(buf) - count, "%s", dt.pm ? "pm" : "am");
    }
    return String(buf);
}

String formatted_date(unsigned long epoch) {
    return formatted_date_time(epoch, true, false);
}

String formatted_time(unsigned long epoch, bool time12 = true, bool sec = false) {
    return formatted_date_time(epoch, false, true, time12, sec);
}