Type > Text Variables > Define...
Then define a variable.
For example, I would use Month and Year as the variable. It makes it easier and more descriptive overall. Depending upon how often the document changes, adding day as well may help.
So....
Note the Preview. That's what the auto-text will look like.
Then, somewhere in your document create a text frame and choose Type > Text Variables > Insert Variable and choose the variable you just created. Set the font, size, style, etc you want.
Now every time you modify the document and save it the variable will be updated after the save. So before outputting, you need to save the file for the variable to be updated.
I usually open a file, make my first change and save - that ensure variables based upon file modifications update. Then continue on with any changes.