As a general rule, medieval artists portrayed the past as their own present. The Virgin Mary as a very respectable craftsman's wife of the year painted, the soldiers at the crucifixion in the armor of the year painted, and so on.
Except when they didn't.
The past is a different country. Things changed more slowly then, but still some artists noticed that the armor on the old effigies in the churches was different from the current state of the art, and likewise for old manuscripts and pattern books and so on.
For example, consider BNF Français 343 Queste del Saint Graal/Tristan de Léonois, a beautiful Milanese manuscript of 1380-1385. At first glance, it seems like a detailed depiction of contemporary fashion, arms and armor.
Yet the artist has actually taken steps to evoke an earlier age, since the text is of Arthurian legend. The knights are repeatedly shown in crested helms that have, by the 1380s, been abandoned except for jousts and tournaments. The pollaxes that were popular weapons for men at arms in the actual 1380s are absent. The knights consistently wear sleeveless tightly fitted coat-armors and jupons over their armor, although artwork done at the same time that wasn't so deliberately aimed at evoking the past often show knights in looser garments with full or partial sleeves.
This sort of deliberate archaicism was not uncommon in art of the 14th and 15th centuries and later. It is well to know what content the artist was depicting, and whether it was far enough in the past to perhaps justify archaic elements. It is also helpful to know of contemporary art where the artist was trying to depict the present or recent past, as a point of comparison