Unix Time stamp creation in C from custom data fields

I wish to create a Unix time stamp. I have a micro-controller with a ble connection that I send the present time to via a GATT connection. I receive the data several integers.

These are Year, Month, Day, Hour, Minute and Second. e.g:

Year = 2020, month = 3, day = 9, hour = 10, minute = 10, second = 10.

I want to convert this to Unix time so I can then increment the value every second via a interrupt based timer. Therefore, what is the correct way to conjoin this data together to form the Unix time. My presumption is as follows:

Create a integer value large enough to store the value called unix time and add the equivalent seconds from the respective time components(year,month etc).

These are the value in seconds

Year_in_seconds = 31,556,952
Month_in_seconds = 2,630,000
Day_in_seconds = 86,400
Hour_in_seconds = 3,600
Minute_in_seconds = 60

Therefore, Unix time =

(nYears * Year_in_seconds)+(nMonths * Month_in_seconds)+(nDays * Days_in_seconds)+(nHours * Hour_in_seconds)+(nMinutes * Minute_in_seconds)+(nSeconds * 1)

Is there anything wrong about my presumptions here?

I don't know how unix time deals with the different number of days in a month as different months have different lengths. Also leap years. So do I need another lookup table or something to make this accurate?

Or what is the correct method for getting the unix time from my set of data?

Solution

This should be accurate up to 2100:

unsigned int tounix (int year, int month, int day, int hour, int min, int sec) {
    unsigned int epoch = 0;
    // These are the number of days in the year up to the corresponding month
    static const unsigned int monthdays[] = { 0, 31, 59, 90, 120, 151, 181, 211, 242, 272, 303, 333 };
    epoch = (year-1970)*86400*365u +    // Account the years
            ((year-1968)/4)*86400 +    // leap years
            monthdays[month-1]*86400 + // days in past full month
            (day-1)*86400u + hour*3600 + min*60 + sec; // days, hours, mins, secs

    if (!(year%4)&&month < 3) // Correct 1 day if in leap year before Feb-29
        epoch -= 86400;

    return epoch;
}

Input is expected as:

Year: 1970 - 2099
Month: 1 - 12
Day: 1 - 31
Hour, Minute, Second: as usual ;)