Looking at chapter 12 in Book of Revelation, we have the following verses [in brown]:
1And a great sign appeared in heaven: A woman clothed with the sun1, and the moon under her feet, and on her head a crown of twelve stars:

3And there was seen another sign in heaven: and behold a great red dragon, having seven heads, and ten horns: and on his head seven diadems: 4And his tail drew the third part of the stars of heaven, and cast them to the earth: and the dragon stood before the woman who was ready to be delivered; that, when she should be delivered, he might devour her son.

5And she brought forth a man child, who was to rule all nations with an iron rod: and her son was taken up to God, and to his throne. 6And the woman fled into the wilderness, where she had a place prepared by God, that there they should feed her a thousand two hundred sixty days.

17And the dragon was angry against the woman: and went to make war with the rest of her seed, who keep the commandments of God, and have the testimony of Jesus Christ.2

Popular constellations have been named for over 3000 years, well before John wrote the Book of Revelation. Only one constellation is that of a 'woman': Virgo.

The constellation Virgo is 'clothed with the sun' at the autumnal equinox: September when the sun crosses the constellation Virgo. The crossing through Virgo takes over a month.

...'the moon under her feet' will occur on September 23rd, 2017. The moon's passing in Virgo's constellation is not unique to Sept 2017 but the 23rd is the only date in this cycle where the moon is 'under her feet'

...'on her head a crown of twelve stars': Atop the constellation Virgo is the constellation Leo made up of 9 stars. September 2017 gives us a unique event that has not yet occurred and will not be repeated: the presence of Mercury, Venus and Mars immediately above the constellation Virgo and alongside the constellation Leo. These three roaming planets and the constellation of Leo with its nine stars makes up the crown of 12 stars on Virgo's head.

The planet Jupiter, our solar system's largest planet, approaches the constellation Virgo in late 2016. In 2016, it is easily visible at Virgo's left shoulder. By September 2017, Jupiter's position is that of exiting Virgo's womb: she brought forth a man child.

The crown of 12 stars made up of Leo's nine stars and three roaming planets, the presence of Jupiter birthing from Virgo's womb, the moon under Virgo's feet and Virgo clothed with the sun...All of these will occur at same time ONLY ONCE: Sept 23rd, 2017.

What we can understand from Revelation 12's great sign is that it is a unique event that gives us a unique point in time.

After September 2017, the 'woman will flee into the wilderness, where she had a place prepared by God, that there they should feed her a thousand two hundred sixty days.

The 1260 days [42 months / 3.5 years] is used elsewhere in the Book of Revelation ( ch11:2-3; ch13:5) as well as in Daniel (ch7:25; ch12:7). In ch.13:5, 42 months is the time of the antichrist's public reign when he has power to rule and persecute.
Does the 1260 days in ch12:6 represent the same period as ch13:5? Not necessarily. Various time frames in the Bible can be staggered, can overlap or follow each other.
Does this mean the 1260 days starts in September 2017? Not necessarily; it could be days, weeks, months later before the 1260 days begins.
There are other sites who attempt to suggest calendar dates for events to follow like the 2nd Coming. I caution to avoid such sites that assign dates as they are mere guesses and may cause confusion and sow disbelief.

What we can take away is that chapter 12 confirms to us that we are in the times of the Book of Revelation.

image showing stars, planets, sun and moon on Sept 23rd, 2017

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