Isaiah uses the phrase “days of east” ( yemey-qedem ) several times in his prophecy (23:7; 37:26; 51:9).  At times, the word “east” by itself is used in context where it seems to have a temporal significance ( 45:21; 46:10).  East is the direction of the sunrise, so “days of east” would presumably early days, morning days, in the morning of the world.  That is the way most translators take the phrase, using “ancient days” or “ancient times.”

Yahweh is the God who speaks “from the east” and who declares “from the east” things that are not yet done (45:21; 46:10).  Yahweh is the God  of the sunset even as the sun is rising.

“East” is also the location of the garden in Eden, the location of the cherubim at the gate of the garden, the location of the gateway to the sanctuaries in the Bible.  ”Days of the east” might thus allude back to the days of Eden, when the God of the East met with man in the garden that was east in Eden.