This answer is given as a supplement to the answer by Lesley to show how the 70 weeks are linked to the 2300 days.
Daniel 9:24 (KJV 1900)
24 Seventy weeks are determined upon thy people and upon thy holy city, to finish the transgression, and to make an end of sins, and to make reconciliation for iniquity, and to bring in everlasting righteousness, and to seal up the vision and prophecy, and to anoint the most Holy.
The word determined comes from the following root:
2852 חָתַךְ [chathak /khaw·thak/] v. A primitive root; TWOT 778; GK 3155; AV translates as “determined” once. 1 to divide, determine. 1A (Niphal) to be determined, be decreed, be settled, be marked out.
Strong, J. (1995). Enhanced Strong’s Lexicon. Woodside Bible Fellowship.
It means "to be divided" or "be marked out" - out of what?
This can be understood when we understand the reason for the prayer of Daniel in Chapter 9. The angel who came to answer Daniel's prayer says in the following:
Daniel 9:22–23 (KJV 1900)
22 And he informed me, and talked with me, and said, O Daniel, I am now come forth to give thee skill and understanding. 23 At the beginning of thy supplications the commandment came forth, and I am come to shew thee; for thou art greatly beloved: therefore understand the matter, and consider the vision.
Understand the matter and consider the vision - what matter and vision is being talked about here?
Daniel 8:26–27 (KJV 1900)
26 And the vision of the evening and the morning which was told is true: wherefore shut thou up the vision; for it shall be for many days. 27 And I Daniel fainted, and was sick certain days; afterward I rose up, and did the king’s business; and I was astonished at the vision, but none understood it.
It is clear that Daniel did not understand the vision of 2300 days. Hence the angel came to explain it to Daniel as an answer to Daniel's prayer. Now the angel says 70 weeks are "determined" or "divided" or "marked out" upon thy people - this is clearly a reference to what Daniel did not understand - the Vision of the Evening and the Morning.
Hence this time period of 70 weeks gives the date for the beginning of the 2300 days.
Concerning the beginning year of 70 weeks:
Daniel 9:25 (KJV 1900)
25 Know therefore and understand, that from the going forth of the commandment to restore and to build Jerusalem unto the Messiah the Prince shall be seven weeks, and threescore and two weeks: the street shall be built again, and the wall, even in troublous times.
The command was to not just build but restore Jerusalem. The command given to Ezra caused restoration of Jerusalem by returning to it a full control of its governance.
Ezra 7:11–26 (KJV 1900)
11 Now this is the copy of the letter that the king Artaxerxes gave unto Ezra the priest, the scribe, even a scribe of the words of the commandments of the LORD, and of his statutes to Israel.
12 Artaxerxes, king of kings, unto Ezra the priest, a scribe of the law of the God of heaven, perfect peace, and at such a time. 13 I make a decree, that all they of the people of Israel, and of his priests and Levites, in my realm, which are minded of their own freewill to go up to Jerusalem, go with thee. 14 Forasmuch as thou art sent of the king, and of his seven counsellers, to inquire concerning Judah and Jerusalem, according to the law of thy God which is in thine hand; 15 And to carry the silver and gold, which the king and his counsellers have freely offered unto the God of Israel, whose habitation is in Jerusalem, 16 And all the silver and gold that thou canst find in all the province of Babylon, with the freewill offering of the people, and of the priests, offering willingly for the house of their God which is in Jerusalem: 17 That thou mayest buy speedily with this money bullocks, rams, lambs, with their meat offerings and their drink offerings, and offer them upon the altar of the house of your God which is in Jerusalem. 18 And whatsoever shall seem good to thee, and to thy brethren, to do with the rest of the silver and the gold, that do after the will of your God. 19 The vessels also that are given thee for the service of the house of thy God, those deliver thou before the God of Jerusalem. 20 And whatsoever more shall be needful for the house of thy God, which thou shalt have occasion to bestow, bestow it out of the king’s treasure house. 21 And I, even I Artaxerxes the king, do make a decree to all the treasurers which are beyond the river, that whatsoever Ezra the priest, the scribe of the law of the God of heaven, shall require of you, it be done speedily, 22 Unto an hundred talents of silver, and to an hundred measures of wheat, and to an hundred baths of wine, and to an hundred baths of oil, and salt without prescribing how much. 23 Whatsoever is commanded by the God of heaven, let it be diligently done for the house of the God of heaven: for why should there be wrath against the realm of the king and his sons? 24 Also we certify you, that touching any of the priests and Levites, singers, porters, Nethinims, or ministers of this house of God, it shall not be lawful to impose toll, tribute, or custom, upon them. 25 And thou, Ezra, after the wisdom of thy God, that is in thine hand, set magistrates and judges, which may judge all the people that are beyond the river, all such as know the laws of thy God; and teach ye them that know them not. 26 And whosoever will not do the law of thy God, and the law of the king, let judgment be executed speedily upon him, whether it be unto death, or to banishment, or to confiscation of goods, or to imprisonment.
As it can be seen clearly that Ezra was empowered by the King to appoint Magistrates and Judges - a privilege only granted to officially appointed authority - as he wills and to execute judgment "whether it be unto death" - the highest form of punishment. Thus Jerusalem began to be restored to her self-governance by this degree.
Nehemiah came later with a letter from the King commanding co-operation of all local forces in building the walls and streets of Jerusalem which is also mentioned in the prophecy of Daniel 9.
But Jerusalem began getting restored in 457 BC by the command of Artaxerxes to Ezra, thus the date
All emboldened text in the scriptures referenced, are added for emphasis by me and are not in the original text.