I know most of the canon was tossed so we are left with the 7 movies, the 2 animated series and acouple of books to draw from.
In any of those sources do we know roughly when the Death Star was built?  The movies tell us any time between 3 and 4.   Did Rebels give a closer time frame to 4?
		
		
	 
Plans for a moon-shaped battle station were first developed by the weaponsmiths of 
Geonosis for use by two malevolent 
Sith Lords, 
Darth Sidious and 
Darth Tyranus, in the waning 
decades of the 
Galactic Republic.
[1] Its design, centered around a superlaser array using massive kyber crystals, was inspired by a technology developed by the ancient 
Sith.
[9]
Ten 
years after the 
Invasion of Naboo, Tyranus was at the head of the 
Confederacy of Independent Systems, a secessionist movement on the brink of the war with the Republic. That year, the Republic ended up sending its newly-acquired 
army of 
clone troopers to Geonosis, which started the 
Clone Wars. The conflict had in fact been provoked by Sidious, who secretely manipulated both sides: he himself led the Republic under his public persona of 
Supreme Chancellor Palpatine, while his 
acolyteTyranus led the Confederacy as 
Count Dooku. With the invasion of Geonosis by the clones, 
Geonosian leader 
Poggle the Lesser turned the designs of their "Ultimate Weapon" over to Dooku for safekeeping, not knowing of the man's alter ego and ties with Sidious.
[1]
As the war raged on, the Ultimate Weapon saw construction over Geonosis. By the time the three-year conflict came to 
an end, workers had partially completed its framework. As the war ended, Darth Sidious got rid of his Confederacy puppets, including his own apprentice, and 
proclaimed the birth of a new, authoritarian 
Galactic Empire, anointing himself 
Emperor. The Ultimate Weapon project was immediately appropriated by the nascent Empire.
[10] Access to Geonosis was restricted, and very few members of the Imperial hierarchy were allowed to know about the project.
[11] Tiaan Jerjerrod, an architect and starship designer from 
Tinnel IV, was enlisted to help build the megaweapon.
[12] Despite construction moving ahead, not all Imperials believed in the viability of the project, with some voicing protest over what would later become known as 'Tarkin's folly.' General 
Jylia Shale was one of many that voiced protest against its development, only to result in her input being marginalized over the coming years.
[13]
	Construction
Edit
			"
The Outer Rim is yours to oversee—and with it, Grand Moff Tarkin, the whole of the mobile battle station project."
			"I will not fail you."
			"It will be a momentous responsibility. For once the battle station is fully operational, you will wield the ultimate power in the galaxy."
			"I don't believe that will ever be the case, my lord."
		
		
			―Wilhuff Tarkin, to Emperor Palpatine
[src]
		
	
http://vignette4.wikia.nocookie.net...th_Star.png/revision/latest?cb=20130206013956
	The Emperor and Darth Vader observe the construction of the Death Star
The construction of the Death Star continued above Geonosis in utmost secrecy, with the Empire utilizing 
construction modules[14] operated by 
Wookiee slave laborers and various other species to complete the project.
[13] To protect the battle station during the long construction phase, a network of bases was established. Initially, supervision of said bases was entrusted to 
Vice Admiral Rancit, who oversaw them all from 
Sentinel Base. However, Rancit was replaced by 
Wilhuff Tarkin at some point after the 
Antar Atrocity. During his tenure as supervisor of the Death Star project, Tarkin had to face many difficulties, from supply problems and attempts at sabotage to missed engineering deadlines. However, Tarkin proved that the Emperor had been right to assign him to that project. As a reward for his efficiency, Tarkin was made the first 
Grand Moff of the Empire, and given greater control over the project.
[8] At some point during its construction, the station was moved from Geonosis to another base, and in order to keep the project a secret, the entire 
Geonosian species was 
wiped out. Three years before the 
Battle of Yavin, one of the modules used during construction was used by 
ISB Agent Kallus to 
ambush a group of rebels, who were on a mission to find out what the Empire was building above the planet, however, their attempt failed and the Rebels managed to escape.
[14] Kyber crystals were also scoured from across the galaxy in order to construct the station, with the Separatist Alliance doing so near the end of the Clone Wars only to have the attempt be 
sabotaged on the Outer Rim world of 
Utapau by 
Jedi Master Obi-Wan Kenobi and 
Jedi Knight Anakin Skywalker,
[15] while a band of 
Lothal rebels managed to 
attack an Imperial supply convoy carrying the crystals five years before the Battle of Yavin, destroying them in the process.
[16]
	Completion and demonstration
Edit
			"
This station is now the ultimate power in the universe."
		
		
			―
Admiral Conan Antonio Motti[src]
		
	
The massive construction project was completed nearly twenty years after its conception during the Clone Wars, and at some point was rechristened as the Death Star.
Imperial Navy pilots and military personnel were eventually assigned to the station shortly before it was revealed to the public, being new enough to still be labeled as classified.
[7] Three weeks later, Imperial forces commanded by Darth Vader 
captured Imperial 
Senator and 
rebel sympathizer Princess 
Leia Organa of 
Alderaan who had received the technical readouts for the Death Star from Rebel agents after a 
major battle, and intended to pass them along to her allies.
[4]
http://vignette4.wikia.nocookie.net...ing-GoM.png/revision/latest?cb=20140417052146
	Alderaan is destroyed by the Death Star
As a high-value prisoner, Organa was moved to a 
detention block aboard the Death Star, where she was interrogated in order to learn the location of the Rebellion's secret base. Faced with her considerable resistance to Imperial interrogation techniques, Tarkin opted to try a different approach: intimidation. In a move calculated to force Organa to reveal the 
Rebel base, Tarkin threatened to use the station's primary weapon to 
destroy her homeworld. Though she ostensibly relented and supplied the rebel base's location as being on 
Dantooine, Tarkin ordered the attack to move forward in order to demonstrate the military power that the Empire now held. In a matter of moments, the Death Star destroyed the planet Alderaan, leaving no survivors.
[4]
Immediately after the planet's destruction, the Star Detroyer 
Devastator left the station and a small Imperial force was sent to investigate Leia's claim under the command of General 
Cassio Tagge.
[17] Upon arrival in the system however, it was discovered that the base was abandoned.
[7] Angered by Leia's lie, Vader and Tarkin conspired to allow Organa to be 
rescued by a group of outlaws, in order to follow them back to the Rebel base themselves.
[4]
http://starwars.wikia.com/wiki/Death_Star