The Hebrew word (transliteration) for day is "yom." It is translated into English as many different words other than day. Here is NASB translation list:
afternoon* (1), age (8), age* (1), all (1), always* (14), amount* (2), battle (1), birthday* (1), Chronicles* (38), completely* (1), continually* (14), course* (1), daily (22), daily the days (1), day (1115), day of the days (1), day that the period (1), day's (6), day's every day (1), daylight* (1), days (635), days on the day (1), days to day (1), days you shall daily (1), days ago (1), days' (11), each (1), each day (4), entire (2), eternity (1), evening* (1), ever in your life* (1), every day (2), fate (1), first (5), forever* (11), forevermore* (1), full (5), full year (1), future* (1), holiday* (3), later* (2), length (1), life (12), life* (1), lifetime (2), lifetime* (1), live (1), long (2), long as i live (1), long* (11), midday* (1), now (5), older* (1), once (2), period (3), perpetually* (2), present (1), recently (1), reigns (1), ripe* (1), short-lived* (1), so long* (1), some time (1), survived* (2), time (45), time* (1), times* (2), today (172), today* (1), usual (1), very old* (1), when (10), when the days (1), whenever (1), while (3), whole (2), year (10), yearly (5), years (13), yesterday* (1).
The Ezekiel 28:15 verse could just have easily have been translated . "Thou was perfect in thy ways from the time that thou wast created, till iniquity was found in thee." Here is the GW translation: "Your behavior was perfect from the time you were created, until evil was found in you."
A "day" in scripture can mean many diferent things. A day for the LORD is as a thousand years. The term may refer to an era (Matt 24:37) or to the span of human history (Gen 8:22), or specify a memorable event (Isa 9:4) or a significant time (Zep 1:14-16). The term often has a metaphorical meaning. Your argument for a 24 hour period definition of "yom" in Ezekiel 28:15 holds no water.
Lucifer, as a Cherub, was shining from continuously being in the presence of God. Scripture says God is light. As I said before, Moses face also shone from being in the presence of the LORD.
There is no scriptual evidence for the premise that angels were created within the time frame of the first chapter of Genesis. There is no mention of God creating the spiritual realm that includes His heavenly home, nor of angels at this time (as I mentioned before). Omission is never a good basis for showcasing evidence. We do know from Scripture, however, that the angels were shouting for joy on the first day in which God created the material earth and heavens (sky and outer space). THAT is the simple truth of Scripture.