std.algorithm can not be used inside pure functions?

[Adam D. Ruppe via Digitalmars-d-learn]

On Saturday, 6 May 2017 at 13:19:17 UTC, Szabo Bogdan wrote:
> a.begin.toISOExtString,

I believe that function is not marked pure if it is a SysTime 
because it needs to pull global timezone info.

What is the type of a.begin?