Not sure I'm understanding correctly.
If you're just using custom fields, then specifying post_type isn't going to help.
If you're working on a site that's going to show events, have you considered using an event management plugin?
I suppose if you're just using custom fields with posts, you could create a category called 'current' and one called 'past' and then use the category switch in your shortcode to only pull in those posts in the current or past categories.
That would work, but would require that you update the categories as the events pass.
Depending on your total requirements, I'd consider an events plugin.
Hope that helps.