parsing a date array

I have an array that contains a number of dates. I want to provide a month i.e 11 and have Appian return the date for the next month.

i.e

List of Timestamp: 12 items
24/04/2018 12:00 BST 
29/05/2018 12:00 BST
26/06/2018 12:00 BST
24/07/2018 12:00 BST
21/08/2018 12:00 BST
18/09/2018 12:00 BST
16/10/2018 12:00 BST
13/11/2018 12:00 GMT
11/12/2018 12:00 GMT
29/01/2019 12:00 GMT
26/02/2019 12:00 GMT
26/03/2019 12:00 GMT

Provide '10' and have as a return 

13/11/2018 12:00 GMT

I've tried using wherecontains to get the index but it only matches if i provide a full string (16/10/2018 12:00 BST ), not a subset. 
Any ideas ?