c++ boost::posix_time::ptime default value

I have a member variable in my class:

class Foo
{
// ...
private:
  boost::posix_time::ptime t;
}

I want to initialize it in the constructor to a well known value such that I know it hasn't been set by the program yet:

Foo::Foo()
   : t(NULL) // doesnt work
{}

But setting it to NULL doesn't work because its not a pointer.

How do I initialize boost::posix_time::ptime to a well known value?