rcp_get_expiration_date()

Note: This is part of the developer docs and is considered custom code.
Unfortunately, we cannot provide support for custom code at this time as we do not have the additional resources that would be necessary to provide support for custom code. 

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rcp_get_expiration_date( $user_id = 0 )
This function returns the given user’s expiration (or renewal) date in a nice format (according to your “date format” settings specified in Settings > General).
Parameters:

$user_id (int) (optional) – ID of the user to check. If omitted or set to 0, the ID of the currently logged-in user will be used.

Return values:

string – ‘none’ if the subscription never expires, or a formatted date.