In the 2009 live-action version of Watchmen, the intro shows - among other things - the original Nite Owl saving Thomas & Martha Wayne from the mugger on that fateful night. In the background, you can clearly see posters advertising what appears to be Batman No. 1, which was published in 1940. There is also a flyer on the opera house playbill called "Die Fledermaus", which is an actual opera by Johann Strauss dating back to 1874, and the opera house is named "Gotham Opera House".
Are these simply easter eggs to Batman, or does the DC character actually exist somewhere in the timeline of Watchmen?
Note: I am not too familiar with the comics version of Watchmen - I've only read it once and that was years ago. I've read that the Watchmen universe was recently brought into the mainstream DC one with 'Rebirth', but I'm looking for an answer that addresses the timeline before that.