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The Metal Goat year starts February 15, not January 1
A birth year alone doesn't fix a Chinese zodiac sign, because the zodiac calendar is lunar and a Gregorian year like 1991 spans parts of two different lunar years. The Hong Kong Observatory's Gregorian-Lunar Calendar Conversion Table for 1991, the government reference for converting between the two calendars, labels the relevant lunar year Xin-wei (辛未) and marks February 15, 1991 as the first day of its first lunar month, the actual start of the new year. Everyone born earlier that winter, January 1 through February 14, 1991, was born under the outgoing year instead, Geng-wu (庚午), Metal Horse; the equivalent table for 1990 puts that prior year's boundary at January 27.
The year doesn't close out on December 31 either. The following year's table shows the next lunar year, Ren-shen (壬申), Water Monkey, opening on February 4, 1992; the day just before that, February 3, closes out Xin-wei's twelfth and final lunar month. That puts the full Metal Goat window at February 15, 1991 through February 3, 1992, a span of 354 days. It's an ordinary lunar year with no leap month inserted, considerably shorter than the 384-day stretch covered on this calendar's 1987 Chinese Zodiac page, where a repeated sixth lunar month drew that Fire Rabbit year out much longer.
Emma Roberts and Jeremy Allen White were born a week apart, on opposite sides of the boundary
A generic year-to-animal chart, the kind that maps a birth year straight onto a zodiac animal without checking where that particular year's lunar new year fell, assigns anyone born anywhere in calendar-year 1991 to the Goat. That's wrong for the year's first 45 days, and two working actors happen to fall on opposite sides of the boundary within a week of each other.
Emma Roberts, the actress known for Scream Queens and American Horror Story, arrived on February 10, 1991, per Wikipedia, five days ahead of the 1991 cutoff. That timing puts her on the earlier side of the calendar boundary, so her actual sign is the outgoing one, Geng-wu, Metal Horse, not the Metal Goat a plain birth-year lookup would suggest.
Jeremy Allen White, the actor known for The Bear and Shameless, arrived on February 17, 1991, also per Wikipedia, just two days past the same line, which makes him a straightforward Metal Goat with nothing to double-check. The two actors' real signs still share something despite landing on opposite sides of that line: 1990's Horse and 1991's Goat sit back to back in the ten-stem cycle, so both carry the same element, Metal, even though the animal differs.
Why Goat years are always Yin, never Yang
Two independent counters combine to name a Chinese zodiac year in full, and they don't run in step. Twelve Earthly Branches supply the animal; ten Heavenly Stems supply the element, paired off two at a time, one Yang and one Yin, for each of the five elements before the cycle rolls to the next. Because twelve and ten share a factor of two, the branches split cleanly into two fixed groups that never cross: Zi, Yin, Chen, Wu, Shen, and Xu only ever draw a Yang stem, while Chou, Mao, Si, Wei, You, and Hai only ever draw a Yin one. Wei carries the Goat, which puts it permanently in the second group, so every Goat year on record, without exception, is Yin.
That groups the Goat with Mao, the 1987 Chinese Zodiac page's Rabbit branch, and sets it apart from Chen and Xu, the branches behind the Dragon and Dog the 1988 Chinese Zodiac and 1994 Chinese Zodiac pages cover. Wikipedia's entries on the Earthly Branches and the sexagenary cycle lay out the identical six-and-six split across the full set of twelve. Within its own Yin group, a Goat year still rotates through five possible stems, Yi, Ding, Ji, Xin, Gui, each one the Yin half of a single element in turn: wood, fire, earth, metal, water. 1991 draws Xin off that list, so this is technically a Yin Metal Goat, though almanacs drop the "Yin" in practice since the polarity never varies for this animal.
1931 and 2051 bracket 1991 by exactly sixty years
Chinese New Year doesn't sit on a fixed date the way December 25 does; it tracks the lunar calendar and can land anywhere from January 21 to February 20. Three consecutive turnovers show how much that window moves: 1990 opened on January 27, 1991 on February 15, and 1992 on February 4, a nineteen-day spread across just two years. That's exactly why a page like this one checks the Observatory's table year by year instead of assuming last year's date carries over. This site's 1992 Chinese Zodiac page covers that same February 4 start in more detail, the line separating anyone born earlier in 1992 as a holdover Metal Goat from the Water Monkeys born after it.
The stem-branch pairing itself, Xin-wei, is rarer still. Ten stems and twelve branches only realign once every 60 years, so the previous Metal Goat year was 1931 and the next is 2051, a full sexagenary cycle in either direction. Every Goat year between those two carries a different element, stepping through the remaining Yin stems in sequence: Ji-wei, Earth, in 1979; Gui-wei, Water, in 2003; Yi-wei, Wood, in 2015; and Ding-wei, Fire, due in 2027, before the sequence circles back to Xin-wei, Metal, in 2051. DayAlmanac's Chinese Zodiac hub covers other years too, including 1987 and 1994.