How to convert `std.datetime.Duration` to seconds of floating point type?

[drug]

On 19.02.2019 19:35, Alex wrote:
> On Tuesday, 19 February 2019 at 16:26:33 UTC, drug wrote:
>>
>> Well, I understand that using floating point values to represent time 
>> internally is a bad idea and I totally agree. But some convenient API 
>> to convert Duration to floating point and vice versa would be useful 
>> because in mechanics for example you often need to express time in 
>> seconds with fractional part. In this regard std::chrono is more 
>> expressive (the only one though, in general std.datetime is much more 
>> powerful than std::chrono).
> 
> If you need a real (double, float) parameter, just use it. Transport the 
> units separately...
Could you elaborate what you mean?
```
	auto point = vec3(1.0, 2.0, 3.0);
	vec3 velocity = ...;

	auto t = dur!"msecs"(500);

	...

	auto new_point = point + velocity * t.total!"msecs"/1e3;
```
How can I use a real parameter and transport units separately here?