Theme in Dominion is a tricky subject. While the game has a vague medieval theme, it is very much secondary to the mechanics. To give you an idea, read the Secret History of Dominion article; the game evolved from a dungeon crawler (your deck was a hero who "leveled up" by gaining cards) and seems to have gone through at least one phase as a castle-building simulator (the card "Farmland" from the Hinterlands expansion was originally called "New Wing"). In many cases the cards have changed names several times, and of course the art is done by various artists, none of whom is Donald X., the game designer. To get an idea of how much difference this makes, look at the pictures of the various Knights from the Dark Ages expansion; trying to come up with a consistent date based on the designs of the various suits of armor is not going to work, and these are all closely related cards! So trying to narrow it down to a specific time period (e.g. 1483 or even the 15th century) is probably an exercise in futility.
Some cards evoke a feeling of the Dark Ages (such as...many of the cards in the Dark Ages expansion) while other cards have more of a Renaissance feel to them. Individual cards have thematic elements (e.g. you "donate" extra cards to the chapel, the witch gives curses, etc.) and related cards tend to have related names (Witch, Sea Hag, and Soothsayer all give out curses, for instance), but the card interactions are frequently not particularly thematic (how do you remodel a Moat into a Remodel? what does that even mean). To the extent cards are linked by theme, it makes sense to look more at the cards in the same expansion; Seaside tends to have more cards dealing with the Sea and Exploration, while Guilds tends to have more cards representing skilled craftsmen and "guilds" (duh).
Long story short, you can assign your expansion to whatever time period works for you. Dominion is much more of a functionalist sort of game; if a card doesn't work, don't make it, no matter how thematic it is.