MonoTime longevity

[Tanel Tagaväli via Digitalmars-d-learn]

I discovered something potentially troublesome in druntime.
Namely, applications that use MonoTime will break if run 18 hours 
after booting, according to a short program I wrote.

Here's my code:

```
import core.time : MonoTime;
auto mt = MonoTime.currTime;
import std.stdio : writeln;
writeln(mt.ticks / mt.ticksPerSecond / 60 / 60); // prints 18 
(hours) shortly after boot
```

`mt.ticksPerSecond` is `1_000_000_000` on my system.
I really hope I'm doing something wrong.