I want to measure the time left before the next action. I have an online game, where I want to measure spawn times.. Time left before next spawn.

```
auto now = std::chrono::system_clock::now();
auto now_c = std::chrono::system_clock::to_time_t(now - std::chrono::hours(12) - std::chrono::minutes(22));

char buf[100] = {0};
std::strftime(buf, sizeof(buf), "%H:%M:%S", std::localtime(&now_c));
return buf;

```

Above you have my actual code.

For example, the last spawn time for a boss is 12 hours and 22 minutes ago.  

I want to measure how much time left before the next spawn (from now).  

I want the time format to be hour:minutes:seconds. What am I missing?

**EDIT**: add clarifications from comments  

The spawn happens everyday at the same time