Except at the start of a phrase, years should be indicated in numbers. Most style guides agree that beginning a sentence with a numerical is bad form, hence years should be spelt out as words at the start of a phrase. This is especially important when writing about ancient history. Words cannot express how much I dislike this outdated rule. It seems like common sense to me that if you're going to begin a sentence with a number, it should be written as a number too. But apparently not! Years are always spelled out at the start of a sentence even when they're first words in the sentence. This is particularly annoying when writing about ancient history because many books refer to years before 1752 as "anno Domini" ("in the year of our Lord"). The AD system was invented by Pope Gregory XIII in 1582 and became popular throughout Europe. Before then years were usually referred to as "of the emperor so-and-so" or some similar phrase. In science texts, years are normally written in full, except for quotations or where they're needed for clarity (e.g. "Jane Smith wrote this book in 1917" doesn't make sense if you don't know it was published in 2017). In academic papers, years are often abbreviated to save space.