The Time of Decision (決断の刻 Ketsudan no Toki, localized as Time for Determination) was an event in BlazBlue: Chronophantasma. It means it was the 'time' to make a 'determination', in other words determining the timeline. This event solidified the existence of the time between 2100 AD to 2200 AD.
Following the Battle of Ragna the Bloodedge, Rachel Alucard invited Ragna the Bloodedge, Celica Ayatsuki Mercury and Noel Vermillion to the ruins of the 5th Hierarchical City "Ibukido", specifically the Grave Marker of Bases. Here, Rachel revealed that the Kiln in the area was Noel's birthplace, explaining her persistent headaches, she further revealed that the caskets inside the area were filled with Dimensional Boundary Contact Prime Fields that were used to create Noel. Afterwards, Rachel revealed that she intended to use the Kiln as a means to send Ragna back to the First Great Magic War. The difficulty of the decision lay with the fact that Ragna held the possibility of dying on the journey, meaning that Rachel's decision was a heavy one. Eventually, she and Ragna agreed for him to be sent back to the past, back to "The Beginning "Zero"".
In order to send Ragna to the past, Rachel commanded Celica to remember the area that Ragna would be sent to with as much clarity as possible, while commanding Noel to Observe Ragna, and recognize his existence. When the two had mustered enough concentration to do their orders, Rachel placed the Tsukuyomi Unit on Ragna so he could survive the ordeal of crossing the Boundary; Ragna then thanked everyone present and jumped into the Kiln after Celica and Noel had gathered their thoughts together. Because of the power used to send Ragna back a century, Rachel lost her ability to use transition magic.
Hades: Izanami later revealed that because of Rachel sending Ragna back to the past, the timeline had been solidified, with no possibilities of the past changing.