Meaning of the two consecutive Colons ::

The :: is part of the calling sequence when you call native .NET classes or get static fields.

In this the .NET class is System.DateTime and the static field is Now.

HTH

You can explore more about these kinds of members in an example like this by deleting the word Now, so all you are left with is

[DateTime]::

and doing tab completion to see what else comes up.  If you want to get a more explicit list of static members on an object check the MSDN documentation for the class or just type the

[ClassName]

followed by

| gm -static

In this case you could type:

[DateTime] | gm -static

To get more on the Now member you could type

[DateTime]::Now | gm -static