Answer first: If your Seiko mod has a day-and-date window, it is almost certainly running a Seiko NH36. If it shows date only (or no date at all), it is an NH35. A Miyota 8-series is the alternative you will find in some slimmer dress builds. All three are genuine Japanese automatic movements - real self-winding mechanicals, not quartz - and most Rosevari Watches builds use one of them (our chronographs use a Seiko meca-quartz instead).
TL;DR
- NH35 - date only, hacking + hand-winding, 24 jewels, ~41h reserve. The workhorse.
- NH36 - adds a day-of-week wheel (day + date), otherwise identical to the NH35.
- Miyota 8215/821A - Citizen-made automatic, slightly thinner, hand-winds; hacking depends on when it was made (added to the series around 2018-2019). Distinctive one-direction rotor.
Why the movement is the first thing to check
The number one question we get is not about price. It is "is this fake?" So let us settle it cleanly: a Seiko mod is a watch built on genuine parts around a genuine movement, with a genuine Seiko-marked dial (never a luxury brand’s name). That is a homage. A counterfeit is a watch with a luxury-brand logo stamped on a dial that did not come from that brand. Rosevari Watches never does the second thing.
The terminology is not ours; it is the industry's. James Enloe set it out in Worn & Wound back in 2013, in what is still the reference piece: "a homage watch is generally thought to be one that 'pays respect' to another watch by using elements of its design or style." Of counterfeits, Enloe wrote: "These watches are made to be deceptive, owners will frequently pass them off as the real thing" (Homage vs. Replica vs. Counterfeit, Worn & Wound, 2 April 2013). Knowing which movement is inside is how you prove to yourself the watch is the real mechanical article.
Learn the fundamentals first in What is a Seiko mod?, then come back here for the movement detail.
The Seiko NH35: the default workhorse
The NH35 (made by Seiko Instruments' movement operation, Time Module Inc, usually shortened to TMI) is the most common automatic in the entire mod world for good reason. It hacks - the seconds hand stops when you pull the crown, so you can set the time to the second. It hand-winds, so you can get it running without shaking it awake. It runs at 21,600 beats per hour, carries 24 jewels, and holds roughly 41 hours of power reserve. The published factory tolerance is "−20~+40 seconds/day" (Caliber Corner, Seiko Caliber NH35), a wide band on paper, but in practice most examples settle far tighter and can be regulated closer.
If your watch shows a date only, this is your movement. And that date-only layout is not a niche: Datejust-style builds, every one of them a date-window watch, are the most-ordered family in our own order book at 82 of the 174 watches built to order since launch (order-book snapshot, 23 August 2026, accessories excluded).
The Seiko NH36: the NH35 with a day wheel
The NH36 is mechanically the same movement with one addition: a day-of-week disc on top of the date. Caliber Corner puts it directly: "The NH35A and NH36A are very similar and both are based on the same Seiko NH family of movements" (Caliber Corner, Seiko Caliber NH36). Same beat rate, same 24 jewels, same ~41 hour reserve, same hacking and hand-winding (a full manual wind is about 55 turns of the crown). It is also, in effect, the unbranded version of the 4R36 calibre Seiko fits to its own watches. If you see both a day and a date window (common on Datejust-style and Explorer-style builds), you are looking at an NH36.
The Miyota 8-series: the Citizen alternative
Miyota is Citizen's movement arm, and the 8215/821A family is its long-running automatic: 21 jewels, 21,600 beats per hour, and approximately 42 hours of running time on the manufacturer's own page. It is a touch thinner than the Seiko NH movements, which is why builders reach for it on slimmer dress cases.
The hacking story needs telling properly, because most articles get it wrong in one direction or the other. For decades the 8-series famously did not hack. Miyota then added a stop-second device to the family around 2018-2019, and not uniformly across every calibre at once (Caliber Corner, Miyota Caliber 8215). Miyota's current published specification for the 8215 lists "Automatic and hand winding" and a "Stop second device" (Miyota, calibre 8215). So a current-production 8215 hand-winds and hacks; an older one hand-winds but may not hack. The one quirk every owner notices either way is the rotor: it winds in one direction only, so it freewheels the other way with a faint wobble some owners love and some do not. It is a proven, reliable calibre that has powered millions of watches.
Full movement spec table
| Spec | Seiko NH35 | Seiko NH36 | Miyota 8215 |
|---|---|---|---|
| Type | Automatic (mechanical) | Automatic (mechanical) | Automatic (mechanical) |
| Maker | Seiko / TMI | Seiko / TMI | Miyota (Citizen) |
| Complication | Date | Day + Date | Date |
| Beat rate | 21,600 bph | 21,600 bph | 21,600 bph |
| Jewels | 24 | 24 | 21 |
| Power reserve | ~41 hours | ~41 hours | ~42 hours |
| Hacking (stop seconds) | Yes | Yes | Current production: yes; older units: no |
| Hand-winding | Yes | Yes | Yes |
| Stated accuracy | -20 / +40 s/day | -20 / +40 s/day | -20 / +40 s/day |
| Best suited to | Divers, tool watches | Day-date dress/field | Slim dress builds |
What Rosevari Watches uses - and why it matters
Most Rosevari Watches builds run a genuine Japanese automatic from this family (our chronographs use a Seiko meca-quartz instead), paired with a sapphire crystal rather than the soft mineral glass you find on cheaper mods. Sapphire resists scratches far better and is a real, verifiable spec advantage at this price. That combination - a real Seiko or Miyota automatic under scratch-resistant sapphire - is the honest heart of the value here. It is what makes these watches get looked at twice.
A quick note on a movement you may see elsewhere: the VK63 is a meca-quartz chronograph movement. It is a smooth, satisfying caliber, but be clear with yourself that it is not an automatic - it runs on a battery with a mechanical chronograph module. If automatic is what you want, stick to the NH or Miyota calibers above. And if a second time zone is the goal, the same NH family offers the NH34 GMT, which we cover in NH34 GMT Movement Explained.
Frequently asked questions
Is the NH35 a real Seiko movement? Yes. The NH35 is manufactured by Seiko Instruments (TMI), Seiko's movement division. It is the same architecture used across countless watches and is a genuine mechanical automatic.
NH35 vs NH36 - which is better? Neither is "better." They are the same movement; the NH36 simply adds a day-of-week wheel. Choose NH36 if you want day + date, NH35 if you only want a date or a cleaner dial.
Why do some mods use Miyota instead of Seiko? The Miyota 8-series is slightly thinner, which suits slim dress cases. It is a reliable Citizen-made automatic that hand-winds; on older units the seconds hand does not stop while you set the time.
Does the Miyota 8215 hack? It depends on when it was made. Miyota added a stop-second device to the 8-series around 2018-2019, and the current published spec for the 8215 lists one. Older production runs do not hack. Every 8215 hand-winds.
Do these movements keep accurate time? Factory tolerance is wide, but most settle within a comfortable daily range and can be regulated tighter. These are mechanical watches, so expect seconds of drift per day, not the near-perfect accuracy of quartz.
What movement is in my Rosevari watch? Check the complication: date only points to an NH35, day + date to an NH36, and our slim dress pieces may use a Miyota. Most are a genuine Japanese automatic under sapphire (our chronographs use a Seiko meca-quartz), backed by a 1-year AU-based warranty.
Ready to see which movement lives in each build? Browse the full range at /collections/all. Every Rosevari Watches piece ships free from Australia, is made to order, and is covered by a 1-year AU-based warranty from A$289; current build timelines are published on the shipping policy page.
Written by Jordan, founder of Rosevari Watches. I build and sell these watches myself, and I would rather tell you exactly what is inside than sell you a mystery.