Re: "Is ngx_current_msec suitable for the date header?"
A quick test shows `ngx_current_msec` is a bit less accurate when compared to `timer->NowMs()`, but I think it would be OK to use it to construct a Date header.

```
[timer->NowMs()][ngx_current_msec]
[1374572765747] [1374572765676]
[1374572765747] [1374572765676]
[1374572765748] [1374572765676]
[1374572765748] [1374572765676]
[1374572765749] [1374572765676]
[1374572765749] [1374572765676]
```