SysTime.add!"days" missing

[Spacen Jasset]

On Monday, 17 March 2014 at 11:52:08 UTC, Vladimir Panteleev 
wrote:
> On Monday, 17 March 2014 at 11:11:26 UTC, Spacen Jasset wrote:
>> I would like to subtract 60 days from a SysTime, but find that 
>> SysTime.add!"days" is not available, unlike 
>> Systime.roll!"days" which is available.
>
> You can do this in a simpler way: t -= 60.days;
>
> SysTime.add likely specializes on durations the length of which 
> varies depending from their starting point, due to the varying 
> number of days in a month or in a year.

Thanks. What devilish magic allows for the syntax 60.days? (how 
does it work)