The Blue Hourglass of the Dragon’s Era was first introduced in chapter 123 of the web novel. Before it appeared the sound of glass shattering reached was heard and a large shock violated visibility. The feeling was similar to being summoned to a wave but it's different in a complicated way.
When the Red Hourglass of the Dragon’s Era tried to be summoned the icon that notified the heroes of the wave the time counter had stopped at 3 days remaining until the next wave and right next to it the Blue Hourglass of the Dragon’s Era had materialized with the number 7 engraved on it.
It is located inside the body of the Spirit Turtle. The size of it is smaller than the hourglass in Melromarc, and it exerts a somewhat light presence inside the body of the spirit turtle. It is not possible to touch the hourglass because it is somewhat intangible.
Blue Hourglass of the Dragon’s Era is most likely the Spirit Turtle’s life span.