Just click on the Series button to the left (the speech balloon with "COMIC" on top) and type in "Omnibus".
Select an Omni or Omni series, such as "The Fantastic Four Omnibus (HC) (2005-2021)". It will list every issue contained within those books.
You will see the first issue in Vol. 1 is a "Reprint from Fantastic Four (I) #1". Click on that and it'll take you to the page devoted to the "Fantastic Four" series. There, you'll see the publication date for that particular issue was 1961/11.
The last two issues in the FF Omni vol. 1 were #30 and Fantastic Four Annual #1. Repeat the process for those and you will have the dates covered by that particular omni, helping you sort all your books chronologically.