The ŌkamiStuck universe is different from both our universe, the Homestuck universes and the universe in which Ōkami takes place in, even though the events of Ōkami and Ōkamiden do have a place within the ŌkamiStuck universe.
So, to help clarify when the events which occur in ŌkamiStuck, both pre-session and in flashbacks, (the timeline of the session itself is separate) I will be providing the timeline of events of the ŌkamiStuck universe.
This timeline is currently not complete and may contain a few inaccuracies, but it is a good way to see how events unfolded. It is also split into four sections: the Creation, which deals with how the ŌkamiStuck universe, as well as the Earth seen in ŌkamiStuck, (also known as Kamiwārudo-hitsu, The World of the Brush Gods) was created; another section which has yet to be named which deals with events after Kamiwārudo-hitsu was completed; the Time of Legends, which starts with Yami's entrance, and covers all of the timeline which is related to the legends of Ōkami; and the New World timeframe, which covers everything up to the present day in the ŌkamiStuck timeline - 2012 ABY.
NOTE: Years will either be shown in SY, (Slick Years - the years after the Genesis Frog of the ŌkamiStuck universe was born) BBY (Before the Battle of Yami) or ABY. (After the Battle of Yami)
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Creation: From 0 SY to ~25,250 SY
0 SY - the Bilious Slick of the universe which OkamiStuck starts in was completed, and the victors of the previous incarnation of SBURB move in.
~100 SY - The creation of the Kotoamatsukami, the first gods created within the new universe.
~25,000 SY - The creation of Izanagi and Izanami.
~25,090 SY - Kamiwārudo-hitsu's creation starts by Izanagi's hand.
~25,100 SY - Izanami 'dies', and is sent to the world of Yomi by the SBURB players.
~25,150 SY - Izanagi tries to bring Izanami back from Yomi, but fails to do so.
~25,160 SY - Izanagi cleanses himself of the impurities of his time in Yomi, leading to the birth of Amaterasu Ōkami and the Brush Gods.
~25,250 SY - Kamiwārudo-hitsu's creation is completed, and Izanagi leaves it in the care of his children, especially Amaterasu.
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???: From ~25,250 SY to 300 BBY
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Legends: From 300 BBY to 1 ABY
300 BBY - The Horrorterror Yami manages to enter the universe thanks to a failed SBURB session which had an entry point close to Kamiwārudo-hitsu.
280 BBY - Yami creates Orochi in preperation for an invasion of the Celestial Plain of Kamiwārudo-hitsu.
225 BBY - The Moon Tribe is all but wiped out by Orochi. Waka uses the Ark of Yamato to flee to the Celestial Plain, not knowing that Yami had hidden himself somewhere inside it as part of his plan.
201 BBY - Orochi attacks the Celestial Plain, forcing many to flee to the Ark to excape. The Brush Gods and Amaterasu joined together for the first, and only, time, transforming into Shiranui Ōkami, and she stayed to fight Orochi.
200 BBY - Shiranui, following the advice of Waka after managing to delay Orochi for long enough for the Ark to flee, drags Orochi down to Kamiwārudo-hitsu. However, at the same time, Yami unleashes his minions in the Ark, leading to the Ark of Yamato Genocide.
190 BBY - the Ark crash-lands on Kamiwārudo-hitsu. Yami creates Lechku, Nechku, Crimson Helm and Blight to assist his plans.
175 BBY - Ninetails encounters Yami, and is offered a posision of power if she helps Yami with his plans. Ninetails accepts, and is granted Oni Island as a base of operations.
130 BBY - The Spider Queen is convinced by Ninetails to help Yami's plans.
100 BBY - Shiranui fights Orochi with the help of Nagi and both Amaterasu and Chibiterasu who visited from the future, but is wounded to the brink of death due to Orochi's and Akuro's attacks. The Brush Gods split from her so that they could live on.
99 BBY - A shrine to Shiranui is errected in Kamiki Village. Amaterasu's spirit would rest in here for nearly 100 years.
0 BBY/ABY - The events of Ōkami take place here.
1 ABY - The events of Ōkamiden take place here.
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New World: From 1 ABY onwards
Between 1994 and 1998 ABY - Some of the ŌkamiStuck players are born.
1997 ABY - Daymon's father stops a group of terrorists, then falls in love with one of the people who were kidnapped, who would end up being Daymon's mother. But when she got pregnant with Daymon, his father was caught red-handed. He was punished by having to work in the Celestial Plain for most of the rest of his life, but the yet-to-be-born baby was to be left alone.
1998 ABY - Daymon Ōkami is born.
Between 1998 and 2000 ABY - The rest of the ŌkamiStuck players are born.
2000 ABY - The celebration of 2000 years since Yami's defeat is interrupted by a messenger who predicts that the end of the world as people knew it would come in twelve years time, but is believed to be lying by the majority of people.
2011 ABY - Daymon Ōkami turns 13, and as a result, has a unprepared growth spurt which gives him his tail. Also, Kamiwārudo-hitsu's Frog Temple is discovered by a group who would later call themselves Skaianet, which attracts the attention of the Brush Gods.
2012 ABY - The main events of ŌkamiStuck take place here.