A Modular Annual Calendar describes the construction where the annual calendar lives on a separate module bolted onto a standalone base movement that, on its own, only tells the time. The module cannot run by itself; it draws power from the base caliber beneath it. The logic is straightforward and the trade has used it for decades: a brand can buy or build one time-only base and fit different complication modules on top, which keeps development cost well below that of an integrated movement. The annual calendar itself knows the difference between 30- and 31-day months and asks for just one manual correction a year, when February turns to March. Patek Philippe's ref. 5035 of 1996, the first annual calendar wristwatch, worked exactly this way: a 315 S QA calendar module on a 315 S automatic base.
You meet the modular approach at very different price points today. The Patek Philippe 5396 builds its date, day, month and moonphase on caliber 324 S QA LU using an architecture of wheels and pinions rather than levers, and current retail sits roughly between $40,000 and $50,000. The power-reserve 5146, with its 24-hour subdial, trades on the secondary market around $41,000 to $69,000 depending on metal. At the accessible end, the Longines Master Collection Annual Calendar runs caliber L897 (an ETA A31.L81 base plus a calendar module) with a 64-hour reserve and lands near $2,425, comfortably the easiest way into the complication. Montblanc's MB 25.09 applies the same thinking higher up, stacking Dubois Depraz chronograph and annual calendar modules on a Sellita base.
When buying, weigh what modularity gives and costs you. On the plus side, proven ETA, Sellita or in-house bases are reliable and widely serviceable, and because the module is a discrete layer, some work can be more accessible. On the minus side, adding a module adds height, so a few modular annual calendars wear thicker than integrated rivals. Always set the date inside the manufacturer's safe window, generally avoiding the late-night hours when the change-over gears are engaged. If you want a daily-wear complication in the $2,000 to $5,000 band, the Longines makes obvious sense; if finishing and collectibility lead your list, the Patek references reward the spend.
Do not read modular versus integrated as a quality ranking. The IWC Portugieser Annual Calendar, introduced in 2015 on the in-house caliber 52850, offers a seven-day reserve and shows month, date and day in three separate semicircular windows at twelve o'clock, a more integrated design philosophy than the budget modular examples. Both routes, done well, run happily for years. What matters is that the module and base are a mature, well-sorted pairing, that the service history is clean, and that you respect the correction procedure.