Difference in days between two dates

I've been trying to write a really basic function to determine the difference (in days) between two dates. I am very new to C++ so as you can imagine I am very happy with my work.
This function is part of a much larger code so I have missed off my includes, but the function does compile without warnings.

Just looking for some constructive criticism that a reasonable beginner can take on board.

I should add, this function takes two dates as strings in British format DD/MM/YYYY.

int dateDiffs(std::string date1, std::string date2){

    int startYear, endYear;
    int startMonth, endMonth;
    int startDay, endDay;

    int startInDays, endInDays;

    std::map daysInMonth;

    daysInMonth[1]=31;
    daysInMonth[2]=28;
    daysInMonth[3]=31;
    daysInMonth[4]=30;
    daysInMonth[5]=31;
    daysInMonth[6]=30;
    daysInMonth[7]=31;
    daysInMonth[8]=31;
    daysInMonth[9]=30;
    daysInMonth[10]=31;
    daysInMonth[11]=30;
    daysInMonth[12]=31;

    startYear = atoi(date1.substr(6,4).c_str());
    startMonth = atoi(date1.substr(3,2).c_str());
    startDay = atoi(date1.substr(0,2).c_str());

    endYear = atoi(date2.substr(6,4).c_str());
    endMonth = atoi(date2.substr(3,2).c_str());
    endDay = atoi(date2.substr(0,2).c_str());

    startInDays = (startYear * 365) + (startMonth * daysInMonth[startMonth]) + startDay;
    endInDays = (endYear * 365) + (endMonth * daysInMonth[endMonth]) + endDay;

    return (endInDays - startInDays);
}

@Snowbody has addressed many important points. I'll just talk about the std::map.

The function initializes daysInMonth each time, which is just unnecessary. You should have it initialized only once elsewhere, and it should also be const since it's not to be modified within the code.

If you have C++11 (which you should), you can initialize the map with an initializer list:

const std::map daysInMonth = { {1, 31},
                                         {2, 28},
                                         {3, 31},
                                         {4, 30},
                                         {5, 31},
                                         {6, 30},
                                         {7, 31},
                                         {8, 31},
                                         {9, 30},
                                         {10, 31},
                                         {11, 30},
                                         {12, 31} };

(it can be formatted in any way you'd like)

You should also use an unsigned type for dates since they cannot be negative.

StackExchange Code Review Q#55054, answer score: 4