A 3135 with a day disc
The Cal. 3155 is the Day-Date version of the Cal. 3135, launched in 1988 alongside the new 18238 / 18039 / 18238 Day-Date 36 references. Architecturally it shares the same 4 Hz balance, free-sprung Microstella regulation, balance bridge (vs cock), and 31-jewel construction as the 3135. The difference is the addition of a day-of-the-week disc above the date wheel, with double Quickset (the crown's middle position cycles either day or date depending on direction). The day disc is the visible signature of the Day-Date and the reason this caliber exists.
In every Day-Date 36 (1988-2015)
The 3155 powered the entire Day-Date 36 production from 1988 through about 2015, across all metal variants (yellow gold 18238, white gold 18239, rose gold, platinum 18206/18296). In 2015 Rolex began the transition to the Day-Date 40 with the new Cal. 3255, which has the Chronergy escapement and 70 h reserve. The Day-Date 36 case (~36 mm) continued as a smaller option but eventually moved to the 3255 family as well. The 3155 is therefore the engine of 27 years of Day-Date 36: the entire "President" era from the late 1980s through the mid-2010s.
Why the Day-Date matters in Rolex history
The Day-Date is Rolex's flagship: gold/platinum only, day-of-the-week in full, signature President bracelet, the watch on the wrist of presidents and Don Drapers. The 3155 was the engine of this status object for most of two and a half decades. Specific to Day-Date functionality: the day disc shows the day name in 26 different languages at customer request (Spanish, Arabic, Greek, Hebrew, Italian, Roman numerals, etc.). This is a 3155-era feature that continues with the 3255.
Reliability and service
The 3155 inherits all the robustness of the 3135. Service intervals around 7-10 years; full overhaul at a Rolex Service Centre runs USD 800-1,200. Parts are common and the caliber is fully serviceable at any independent watchmaker certified by Rolex. The day-wheel mechanism adds slightly to service complexity but is itself a known and well-engineered design that has not generated significant failure patterns.
Where it sits
In the modern Rolex lineup the 3155 is now discontinued in current production but remains in heavy circulation on the secondary market. A 1990s-2000s Day-Date 36 in yellow gold trades at USD 12,000-20,000 (lower than equivalent steel Sub of the same era because supply is large and the market for solid-gold dress watches is narrower). The 18039/18238 references with their 3155 movements are prized for their classic Day-Date proportions, which many collectors prefer to the 40 mm modern version.