Converting local time from CTS to UTC time

[Mike Austin (LPI)]

Hi,

I'm currently reading the local time from a client via the CTS over BLE, but need to convert that in my nRF52832 to UTC time.  Is there some API's available that do this?  I can't seem to find any, and was thinking "surely, someone has done this before?"

Cheers,

Mike

[Simon]

Hello Mike,

Should we continue the discussion in  Getting Time Zone and Daylight Savings info out of CTS ?

Best regards,

Simon

[Mike Austin (LPI)]

OK, ended up sorting this out for myself.  Below is my code.  I'm sure someone could make this more efficient, but I'm a hardware guy so just happy it works! Haha.

At the moment I'm just hard coding the values for .timeZone and .dst into my bt_cts_local_time structure, as I've not yet been able to extract this from the CTS.  Plus it made it easier for me to test all the various scenarios.

#define DAYS_IN_WEEK 7
#define MONTHS_IN_YEAR 12
#define MINS_IN_HOUR 60
#define HOURS_IN_DAY 24
#define JANUARY 1
#define FEBRUARY 2
#define MARCH 3
#define DECEMBER 12

/**
 * @brief CTS Local Time structure
 *
 * @param timeZone Offset from UTC in hours
 * @param dst Daylight Savings Time: 1 = active, 0 = inactive
 */
struct bt_cts_local_time
{
    int8_t timeZone;           /**< Current Time Zone */
    uint8_t dst;               /**< Daylight Saving Time value */
} ;

struct bt_cts_current_time local_time, utc_time;
struct bt_cts_local_time local_time_info;

uint8_t daysInMonth[13] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

// Convert time from Current Time Service to UTC time
// cts_current_time = time from CTS
// cts_local_time = local time info (time zone and DST) from CTS
// utc_time = time converted to UTC
void convert_to_utc_time (struct bt_cts_current_time *cts_current_time, struct bt_cts_local_time *cts_local_time, struct bt_cts_current_time *utc_time)
{
    int8_t  minOffset = 0,
            hourOffset = 0,
            dayOffset = 0,
            monthOffset = 0,
            yearOffset = 0,
            days_in_current_month = 0,
            days_in_previous_month = 0;
    uint8_t week_day,
            day,
            month,
            hour,
            min;
    uint16_t year;
    bool isLeapYear;

    week_day = cts_current_time->exact_time_256.day_of_week;
    day = cts_current_time->exact_time_256.day;
    month = cts_current_time->exact_time_256.month;
    hour = cts_current_time->exact_time_256.hours;
    min = cts_current_time->exact_time_256.minutes;
    year = cts_current_time->exact_time_256.year;
    
    /* Calculate time offset due to local time zone and daylight savings */
    // .timeZone and .dst are expressed in multiples of 15 minutes
    // Negative offset means local time is behind UTC, positive offset means local time is ahead of UTC
    hourOffset = (int8_t)((cts_local_time->timeZone/4) + (cts_local_time->dst/4));
    minOffset = (int8_t)((cts_local_time->timeZone%4) + (cts_local_time->dst%4))*15;
    
    if (year%4 == 0) {
        isLeapYear = true;
        if (month == FEBRUARY) {
            days_in_current_month = daysInMonth[FEBRUARY]+1;
        }
        else {
            days_in_current_month = daysInMonth[month];
        }
        if (month == MARCH) {
            days_in_previous_month = daysInMonth[FEBRUARY]+1;
        }
        else if (month == JANUARY) {
            days_in_previous_month = daysInMonth[DECEMBER];
        }
        else {
            days_in_previous_month = daysInMonth[month-1];
        }
    }
    else {
        isLeapYear = false;
        if (month == JANUARY) {
            days_in_previous_month = daysInMonth[DECEMBER];
        }
        else {
            days_in_previous_month = daysInMonth[month-1];
        }
        days_in_current_month = daysInMonth[month];
    }

    /* Update mintues */
    if (minOffset < 0) { 
        //Local behind UTC - increase minutes  
        if ((min + abs(minOffset)) > MINS_IN_HOUR) {
            ++hourOffset;
            min = (min + abs(minOffset)) - MINS_IN_HOUR;
        }
        else {
            min = (min + abs(minOffset));
        }
    }
    else if (minOffset >= 0) {
        //Local ahead of UTC - reduce minutes  
        if (min < minOffset) {
            --hourOffset;
            min = MINS_IN_HOUR + min - minOffset;
        }
        else {
            min = min - minOffset;
        }
    }

    /* Update hours */
    if (hourOffset < 0) {
        //Local behind UTC - increase hours 
            if ((hour + abs(hourOffset)) > HOURS_IN_DAY) {
            ++dayOffset;
            hour = (hour + abs(hourOffset)) - HOURS_IN_DAY;
        }
        else {
            hour = (hour + abs(hourOffset));
        }
    }
    else if (hourOffset >= 0) {
        //Local ahead of UTC - reduce hours
        if (hour < hourOffset) {
            --dayOffset;
            hour = HOURS_IN_DAY + hour - hourOffset;
        }
        else {
            hour = hour - hourOffset;
        }
    }
    

    /* Update day of month */
    if (dayOffset >= 0) {
        if ((day + dayOffset) > days_in_current_month) {
        ++monthOffset;
        day = (day + dayOffset) - days_in_current_month;
    }
    else {
        day = day + dayOffset;
    }
    }
    else if (dayOffset < 0) {
        if (day <= abs(dayOffset)) {
            --monthOffset;
            day = days_in_previous_month + day + dayOffset;
        }
        else {
            day = day + dayOffset;
        }
    }

    /* Update month */
    if (monthOffset >= 0) {
        if ((month + monthOffset) > MONTHS_IN_YEAR) {
        ++yearOffset;
        month = (month + monthOffset) - MONTHS_IN_YEAR;
    }
    else {
        month = month + monthOffset;
    }
    }
    else if (monthOffset < 0) {
        if (month <= abs(monthOffset)) {
            --yearOffset;
            month = MONTHS_IN_YEAR + month + monthOffset;
        }
        else {
            month = month + monthOffset;
        }
    }
    
    /* Update day of week */
    if (dayOffset >= 0) {
        if ((week_day + dayOffset) > DAYS_IN_WEEK) {
            week_day = (week_day + dayOffset) - DAYS_IN_WEEK;
        }
        else {
            week_day = week_day + dayOffset;
        }
    }
    else if (dayOffset < 0) {
        if (week_day <= abs(dayOffset)) {
            week_day = DAYS_IN_WEEK + week_day + dayOffset;
        }
        else {
            week_day = week_day + dayOffset;
        }
    }
 
    /* Set UTC time */
    utc_time->exact_time_256.seconds = cts_current_time->exact_time_256.seconds;
    utc_time->exact_time_256.minutes = min;
    utc_time->exact_time_256.hours = hour;
    utc_time->exact_time_256.day_of_week = week_day;
    utc_time->exact_time_256.day = day;
    utc_time->exact_time_256.month = month;
    utc_time->exact_time_256.year = year + yearOffset;

}

Cheers,

Mike

[Simon]

There you go. I'm glad you found a solution that works for you.

Best regards,

Simon