In the early 1900's and well into the 1950's and 60's, many companies did not keep detailed records like they do now. Or, they did but their archives were destroyed or lost for a variety of reasons. Luckily, MGM kept detailed records (obsessively so in some cases) and most of these have survived. So many dates and events during Judy's MGM years are well documented.
What you will see in these timelines are dates taken from newspaper archives, studio archives, books, CD liner notes, home video commentary tracks, documentaries, and other websites, as well as a variety of other sources. Some dates are disputed, and since most if not all of the participants or witnesses to some events are gone, a best guess estimate is used. Some sources conflict with others, creating more questions than answers. All of these approximate dates are noted with "date approx."
If you have more information or corrections, please email me.
For even more detailed timelines, check out Scott Schechter's fantastic book "Judy Garland The Day-by-Day Chronicle of a Legend." This book is not a "biography" but exactly what the title says: A Day-by-Day Chronicle of Judy's life. With access to the Arthur Freed archives in the UCLA Library, as well as a multitude of other sources, Scott picks up where The Judy Room timelines leave off - and provides even more detailed information than you'll find here. Nice pictures too. For people who enjoy raw data this is the book for you!