# How to convert a UTC & amp; time for a time_t in C ++?

I want to convert a UTC date & time given in numbers for year, month, day, etc. to a time_t. Some systems offer functions like `mkgmtime` or `timegm` for this purpose but that is not standard and does not exist on my Solaris system.

The only solution I have found so far involves setting the local time zone to UTC with setenv and then call `mktime`. However this approach is not thread-safe, slow, not portable and even generates a memory leak on my system.

I have also seen approaches that tried to determine the current UTC offset using `gmtime` and then adding that to the result of `mktime`. But as far as I have seen all those approaches had gaps. After all, the conversion from the local time to UTC is not unique.

What do you think is the best solution?

I have decided to implement my own version of mkgmtime and it was easier than I thought.

``````const int SecondsPerMinute = 60;
const int SecondsPerHour = 3600;
const int SecondsPerDay = 86400;
const int DaysOfMonth[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

bool IsLeapYear(short year)
{
if (year % 4 != 0) return false;
if (year % 100 != 0) return true;
return (year % 400) == 0;
}

time_t mkgmtime(short year, short month, short day, short hour, short minute, short second)
{
time_t secs = 0;
for (short y = 1970; y < year; ++y)
secs += (IsLeapYear(y)? 366: 365) * SecondsPerDay;
for (short m = 1; m < month; ++m) {
secs += DaysOfMonth[m - 1] * SecondsPerDay;
if (m == 2 && IsLeapYear(year)) secs += SecondsPerDay;
}
secs += (day - 1) * SecondsPerDay;
secs += hour * SecondsPerHour;
secs += minute * SecondsPerMinute;
secs += second;
return secs;
}
```
```

My main concern was that `mkgmtime` must be consistent with `gmtime`. Such that `gmtime(mktime(t))` returns the original input values. Therefore I have compared the results for all multiples of 61 between 0 and MAX_INT for time_t and they are indeed equal (at least on my system). Therefore the above routine is correct.

This outcome also means that the C library does not take leap seconds into account, which is a bad thing in itself but good for my purpose. The two functions will stay consistent for a long time. To be absolutely sure, my Timestamp class that uses this function always performs a quick check on program start and proves the consistency for a couple of meaningful values.