How to union tables?

Hi,

In my case there are 12 identical tables for each month (the content is different, of course).

My goal is to create a filter or slicer by months.

All tables have a DATE/TIME column 'CreatedOn' (e.g 3.10.2020 10:11:00) and a TIME column 'Working time' (e.g 7:45)

In January file there are calulated measures like Working hours (sec), Working hours (h), Duration (h) which are calulated from 'Working time'

For filtering purposes I want to union all 12 tables, but PowerBI says all tables must have equal number of columns. 

Is it true that I have to copy those 3 measures to each 12 tables??

Is there any other way to combine those tables?

My goal is to get a filter or slicer by moths.

All help is much appreciated!