Does this (Revelation 9:15) contextually deal with a 'day for a year' [5] , or rather does it specifically deal with the sanctuary and it's Feast Time [3]?

Let's look at the nearest context (Revelation 9:13-15) and expand from there:

Rev 9:13? And the sixth angel sounded, and I heard a voice from the four horns of the golden altar which is before God,?
Rev 9:14? Saying to the sixth angel which had the trumpet, Loose the four angels which are bound in the great river Euphrates.?
Rev 9:15? And the four angels were loosed, which were prepared for an hour, and a day, and a month, and a year, for to slay the third part of men.?

Revelation 9:13,

?... I heard a voice from the four horns of the golden altar which is before God,?

What language is this? ?four horns?, ?golden altar?, and ?before God?? It is Heavenly Sanctuary language:

Psalms 77:13 KJB - Thy way, O God, is in the sanctuary: who is so great a God as our God?

Whose ?voice? was at the ?four horns? of ?the golden altar which is before God??

It is Jesus Christ, the Great High Priest, before the Father:

Revelation 8:3 KJB - And another angel came and stood at the altar, having a golden censer; and there was given unto him much incense, that he should offer it with the prayers of all saints upon the golden altar which was before the throne.

Revelation 8:4 KJB - And the smoke of the incense, which came with the prayers of the saints, ascended up before God out of the angel's hand.

Revelation 8:5 KJB - And the angel took the censer, and filled it with fire of the altar, and cast it into the earth: and there were voices, and thunderings, and lightnings, and an earthquake.

Since we are looking at the Heavenly Sanctuary, since it is ?before God? [Revelation 4:2,5, 8:2,4, 11:16 KJB, etc.], who can we know which ?altar? it is?

How many ?altar[s]? are in the Sanctuary? There are two.

[1] The ?altar? of burnt offering/sacrifice/slaughter, and,

[2] the ?altar? of incense.

How do we know which of the two it is? By the description, ?golden?.

The ?altar? of burnt offering/sacrifice/slaughter was made of brass, called the ?brasen? altar. This was the ?altar? of the courtyard, the ?altar? of the earth. Even the Cross.

Exodus 27:1 KJB - And thou shalt make an altar of shittim wood, five cubits long, and five cubits broad; the altar shall be foursquare: and the height thereof shall be three cubits.

Exodus 27:2 KJB - And thou shalt make the horns of it upon the four corners thereof: his horns shall be of the same: and thou shalt overlay it with brass.

Exodus 27:3 KJB - And thou shalt make his pans to receive his ashes, and his shovels, and his basons, and his fleshhooks, and his firepans: all the vessels thereof thou shalt make of brass.

Exodus 27:4 KJB - And thou shalt make for it a grate of network of brass; and upon the net shalt thou make four brasen rings in the four corners thereof.

Exodus 27:5 KJB - And thou shalt put it under the compass of the altar beneath, that the net may be even to the midst of the altar.

Exodus 27:6 KJB - And thou shalt make staves for the altar, staves of shittim wood, and overlay them with brass.

Exodus 27:7 KJB - And the staves shall be put into the rings, and the staves shall be upon the two sides of the altar, to bear it.

Exodus 27:8 KJB - Hollow with boards shalt thou make it: as it was shewed thee in the mount, so shall they make it.

Exodus 38:30 KJB - And therewith he made the sockets to the door of the tabernacle of the congregation, and the brasen altar, and the brasen grate for it, and all the vessels of the altar,

Exodus 39:39 KJB - The brasen altar, and his grate of brass, his staves, and all his vessels, the laver and his foot,

The ?golden altar? is the ?altar? of ?incense?, in the Holy Place, being associated with the 2nd veil and the Most Holy Place:

Exodus 37:25 KJB - And he made the incense altar of shittim wood: the length of it was a cubit, and the breadth of it a cubit; it was foursquare; and two cubits was the height of it; the horns thereof were of the same.

Exodus 37:26 KJB - And he overlaid it with pure gold, both the top of it, and the sides thereof round about, and the horns of it: also he made unto it a crown of gold round about.

Exodus 37:27 KJB - And he made two rings of gold for it under the crown thereof, by the two corners of it, upon the two sides thereof, to be places for the staves to bear it withal.

Exodus 37:28 KJB - And he made the staves of shittim wood, and overlaid them with gold.

Exodus 37:29 KJB - And he made the holy anointing oil, and the pure incense of sweet spices, according to the work of the apothecary.

Exodus 39:38 KJB - And the golden altar, and the anointing oil, and the sweet incense, and the hanging for the tabernacle door,

Exodus 40:5 KJB - And thou shalt set the altar of gold for the incense before the ark of the testimony, and put the hanging of the door to the tabernacle.

Exodus 40:26 KJB - And he put the golden altar in the tent of the congregation before the vail:

Exodus 40:27 KJB - And he burnt sweet incense thereon; as the LORD commanded Moses.

Leviticus 4:7 KJB - And the priest shall put some of the blood upon the horns of the altar of sweet incense before the LORD, which is in the tabernacle of the congregation; and shall pour all the blood of the bullock at the bottom of the altar of the burnt offering, which is at the door of the tabernacle of the congregation.

1 Chronicles 28:18 KJB - And for the altar of incense refined gold by weight; and gold for the pattern of the chariot of the cherubims, that spread out their wings,

2 Chronicles 4:19 KJB - And Solomon made all the vessels that were for the house of God, the golden altar also, and the tables whereon the shewbread was set;

Revelation 8:3 KJB - And another angel came and stood at the altar, having a golden censer; and there was given unto him much incense, that he should offer it with the prayers of all saints upon the golden altar which was before the throne.

Revelation 8:4 KJB - And the smoke of the incense, which came with the prayers of the saints, ascended up before God out of the angel's hand.

Hebrews 9:3 KJB - And after the second veil, the tabernacle which is called the Holiest of all;

Hebrews 9:4 KJB - Which had the golden censer, and the ark of the covenant overlaid round about with gold, wherein was the golden pot that had manna, and Aaron's rod that budded, and the tables of the covenant;

The ?four horns? on the ?golden? ?altar? of ?incense?, were used specifically in the Day of Atonement, ?once? in the year:

Exodus 30:1 KJB - And thou shalt make an altar to burn incense upon: of shittim wood shalt thou make it.

Exodus 30:2 KJB - A cubit shall be the length thereof, and a cubit the breadth thereof; foursquare shall it be: and two cubits shall be the height thereof: the horns thereof shall be of the same.

Exodus 30:3 KJB - And thou shalt overlay it with pure gold, the top thereof, and the sides thereof round about, and the horns thereof; and thou shalt make unto it a crown of gold round about.

Exodus 30:4 KJB - And two golden rings shalt thou make to it under the crown of it, by the two corners thereof, upon the two sides of it shalt thou make it; and they shall be for places for the staves to bear it withal.

Exodus 30:5 KJB - And thou shalt make the staves of shittim wood, and overlay them with gold.

Exodus 30:6 KJB - And thou shalt put it before the vail that is by the ark of the testimony, before the mercy seat that is over the testimony, where I will meet with thee.

Exodus 30:7 KJB - And Aaron shall burn thereon sweet incense every morning: when he dresseth the lamps, he shall burn incense upon it.

Exodus 30:8 KJB - And when Aaron lighteth the lamps at even, he shall burn incense upon it, a perpetual incense before the LORD throughout your generations.

Exodus 30:9 KJB - Ye shall offer no strange incense thereon, nor burnt sacrifice, nor meat offering; neither shall ye pour drink offering thereon.

Exodus 30:10 KJB - And Aaron shall make an atonement upon the horns of it once in a year with the blood of the sin offering of atonements: once in the year shall he make atonement upon it throughout your generations: it is most holy unto the LORD.

Leviticus 16:18 KJB - And he shall go out unto the altar that is before the LORD, and make an atonement for it; and shall take of the blood of the bullock, and of the blood of the goat, and put it upon the horns of the altar round about.

Revelation 9:13 sets the stage, and imagery for the timeframe of the Day of Atonement. Let us continue and see further. Other items (vs 13-15) we can identify from scripture, but let me jump ahead a moment to verse 15, in the latter half, to the 'time' portion:

Rev 9:15? And the four angels were loosed, which were prepared for an hour, and a day, and a month, and a year, for to slay the third part of men.?

?... which were prepared ...?

To be ?prepared? is to be ?ready? ahead of time [Psalms 9:7; Jonah 1:17, 4:6,7,8; 1 Corinthians 2:9 KJB], ?waiting? for a specific moment of time to be of service and use [2 Timothy 2:21 KJB].

?... for ...?

The word ?for? means, the reason, the purpose [see Ruth 2:16; Nehemiah 8:4; Acts 26:16, etc. KJB]. These ?four angels? have specific purpose and function, a definite message.

?... an hour, and a day, and a month, and a year ...?

The moment of time, that the Day of Atonement came, when the ?golden altar? and the ?four horns? were to be used by the Great High Priest, Jesus Christ in the Heavenly Sanctuary. There are two places in scripture in which an ?hour?, ?day?, ?month? and ?year? are known.

[1] Passover [1 Corinthians 5:7 KJB]; evening, going down of the sun [the 9th ?hour?, 3 PM], 14th day of the 1st Month and 6th Day of the week, the 1st Month Abib, and the year AD 31, the midst of the 70th week of Daniel.

[2] Day of Atonement:

?... an hour ...?, is a specific hour:

The ?hour of his judgment is come?:

Revelation 14:6 KJB - And I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people,

Revelation 14:7 KJB - Saying with a loud voice, Fear God, and give glory to him; for the hour of his judgment is come: and worship him that made heaven, and earth, and the sea, and the fountains of waters.

?... and a day ...?, is a specific day:

The ?tenth? day, the ?day? of atonement [aka: 22nd of October]:

Leviticus 23:27 KJB - Also on the tenth day of this seventh month there shall be a day of atonement: it shall be an holy convocation unto you; and ye shall afflict your souls, and offer an offering made by fire unto the LORD.

Leviticus 23:28 KJB - And ye shall do no work in that same day: for it is a day of atonement, to make an atonement for you before the LORD your God.

Leviticus 23:29 KJB - For whatsoever soul it be that shall not be afflicted in that same day, he shall be cut off from among his people.

Leviticus 23:30 KJB - And whatsoever soul it be that doeth any work in that same day, the same soul will I destroy from among his people.

Leviticus 23:31 KJB - Ye shall do no manner of work: it shall be a statute for ever throughout your generations in all your dwellings.

Leviticus 23:32 KJB - It shall be unto you a sabbath of rest, and ye shall afflict your souls: in the ninth day of the month at even, from even unto even, shall ye celebrate your sabbath.

?... and a month ...?, is a specific month:

The ?seventh month?, the month ?Ethanim? [aka: Tishri, October]:

Leviticus 23:27 KJB - Also on the tenth day of this seventh month there shall be a day of atonement: it shall be an holy convocation unto you; and ye shall afflict your souls, and offer an offering made by fire unto the LORD.

1 Kings 8:2 KJB - And all the men of Israel assembled themselves unto king Solomon at the feast in the month Ethanim, which is the seventh month.

2 Chronicles 5:3 KJB - Wherefore all the men of Israel assembled themselves unto the king in the feast which was in the seventh month.

?... and a year ...?, is a specific year:

The year AD 1843/44, the end of the 2,300 day/years (and 1,335):

Exodus 30:10 KJB - And Aaron shall make an atonement upon the horns of it once in a year with the blood of the sin offering of atonements: once in the year shall he make atonement upon it throughout your generations: it is most holy unto the LORD.

Daniel 8:13 KJB - Then I heard one saint speaking, and another saint said unto that certain saint which spake, How long shall be the vision concerning the daily sacrifice, and the transgression of desolation, to give both the sanctuary and the host to be trodden under foot?

Daniel 8:14 KJB - And he said unto me, Unto two thousand and three hundred days; then shall the sanctuary be cleansed.

Revelation 10:6 KJB - And sware by him that liveth for ever and ever, who created heaven, and the things that therein are, and the earth, and the things that therein are, and the sea, and the things which are therein, that there should be time no longer:

Revelation 9:13-15 is not contextually linked to use the day for a year. It is contextually linked to the sanctuary and the Festal time.

We can look at more of the symbols therein as needed, like the last portion of vs 15, in the 1/3rd, etc, or in "Euphrates", etc. None of which is dealing with Mohammad, Islam, Turks, Ottoman Empire, Sultanies, 391 years and 15 days, or anything which Josiah Litch (and others) incorrectly identified etc.