The timeline for the DC Animated Movie Universe isn’t thoroughly explained, and the franchise has numerous continuity errors with the New 52 comic-book series and has also excluded popular characters. With only 15 films, one feature film, one short, one cameo film and 2 comic-book spin-offs this is the timeline for this film Franchise.
First: jl
Justice League: The Flashpoint Paradox
This film deserves the top spot as the events are what ultimately leads to the rebooted timeline. A calendar on the wall in timeline indicates its November, while at the end when Barry Allen returns to the new timeline the calendar reads October.
Justice League War
These events are set in November as that's when High School State Final's occur. It's crystal clear that Flash has no idea about the flashpoint and the former timeline, as this film takes place in a point in time before Barry wakes up to give Bruce the letter sent to him by his father in the Flashpoint timeline.
Son of Batman
A chart shows up to presumably 2013, not much information is given.
Justice League: Throne of Atlantis
On a phone call, Steve Trevor corrects the person by saying that the team name was changed to Justice League the last week. But it's hinted that these events are the second time the Justice League are going to 'save the world'. Even further Diana is still getting used to the civilian thing, she and Clark were on their first date, Cyborg is still mad at his dad and Orm hatching this plan in the post-credit scene of Justice League: War. This theory was confirmed by Heath Corson, so to conclude it's been a little more than a week.
Nightwing and Robin
This short was set in an early portion of the events of Justice League: Throne of Atlantis, which is also included as a bonus feature on a DVD or home video. Ultimately confirming the Son of Batman took place within that week, which also confirms that this takes place in November 2013.
Batman vs. Robin
This film is set during late snowfall and a dialogue indicates that it's been months since the events of Son of Batman, and Damian is still 10 year's old. Bruce's relationship with Samantha Vanever had been going on for 3 months, which would have begun after Talon staring observing the new Robin as seen in the Nightwing and Robin short. So with all these facts lined up, this would mean that the film takes place in late February to early March.
Bad Blood
Batman: Bad Blood
There is no confirmed date but it's mentioned that the Heretic has been around for 10 months. Oh and this quip:
Talia: How is it that I ever loved either of you?
Batman: I don't think you ever did.
Talia: Oh, Beloved, you have no idea. I have thought of little else for the last twelve years. But nothing—Not my passion, not the death of my father, not even our child—could bring you to my side. I know, now, I will never have you in this life. But perhaps, in death.
Justice League vs. Teen Titans
Damian: So this is my punishment, enduring a bunch of school children?
Dick: This is not punishment. Hopefully you'll learn to be part of a team and they're not children, they're teenagers.
Damian: Worse.
Given Damian's response it's safe to assume that he isn't a teenager yet. A newspaper date of it's sequel 'the Judas Contract' is dated somewhere in 2016, making this take place in 2015.
Suicide Squad: Hell to Pay
With Professor Zoom's appearance, it's possible that this film takes place in October 2015, which would make the ending scene take place at that time.
Suicide Squad: Hell to Pay (Comic)
It's possible that it takes place a few weeks after the events of Suicide Squad: Hell to Pay taking into consideration over Knockout's hospitalization and the fact that Digger Harkness black eye and bruises have healed up.
Justice League Dark
The film occurs an indeterminate amount of time after Suicide Squad: Hell to Pay. The League has expanded and continues to expand its roster, indicating it's at least a couple of years since the League formed in Justice League: War. It’s stated Constantine: City of Demons: The Movie is a few years after the events of this film, placing it in late 2015 at the earliest.
Teen Titans: The Judas Contract
The film picks up a year after the events of Justice League vs Teen Titans, making the prologue to be set in 2011. A newspaper states "the treasury bond heavy quarter of 2016", making it something in late 2016 which would also make Justice League vs Teen Titans taking place in 2015.
Teen Titans Go! vs. Teen Titans (cameo)
Dick Grayson is wearing his Teen Titans The Judas Contract uniform, whereas in Batman: Hush, he’s wearing a new suit, suggesting that his multiverse adventure is set before the events of that film, at least from the DC Animated Movie Universe perspective.
The Death of Superman
The League has a board meeting discussing their operating budget for the upcoming fiscal year, which is typically the last day of either March, June, September or December. Batman mentions that Damian is currently in boarding school, which narrows down the month to be either March or September, as schools would be out in both late June and December. Due to Reign of the Supermen being placed, a couple of months before the Justice League Dark: Apokolips War prologue took place, it would make sense for it to be placed in March 2018 though the Justice League started in November.
The Death of Superman: Part 1 (Comic)
The Death of Superman - Part 1 comic takes place during and after The Death of Superman.
Reign of the Supermen
Takes place six months after The Death of Superman so it’s accurately confirmed that the movie occurs in September 2018.
Constantine: City of Demons
Is dated as occurs in late October to early November 2018 based on a newspaper date.
Batman: Hush
Confirmed to take place in early November 2018.
Wonder Woman: Bloodlines
The prologue takes place about a few months prior to the events of Justice League: War, It's confirmed to take place after Batman: Hush making the film set in late November 2018.
Justice League Dark: Apokolips War
The prologue takes place 2 years prior to the film, the prologue would be dated sometime during November-December 2018. So the film takes place sometime in 2020.
Constantine: The House of Mystery
Post-Script Season: Acts as this with respect to the DCAMU, given it takes place after the Grand Finale in Apokolips War. Really 700 Years Old: Constantine mentions that he has been trapped in the House for centuries due to the loop he was in.