My goal is to determine expiry of an item to when it was acquired(bought) and when it is sold.There is a TTL value associated with each of the item.

I am doing following :

```
time_t currentSellingTime;
long currentSystemTime = time(&currentSellingTime); // this gives me epoch millisec of now()

long TTL = <some_value>L;
long BuyingTime = <some_value> // this is also in epoch millsec

if(currentSystemTime >  TTL+BuyingTime))
{
//throw exception 
// item is expired
} 

```

My question is how to sum two epoch millisec and compare it with another epoch millsec in C++