Display all the items created in current month

In calculated fields you can use the following formulas to get first day of month and last day of month.

First Day of Current Month Formula=DATE(YEAR([Created]), MONTH([Created]), 1) 

Last Day of Current Month Formula =DATE(YEAR([Created]), MONTH([Created])+1,1)-1 

Current Month:

Filter: First day <[Today]

          Last Day>[Today]