The 79830RB is the Black Bay GMT, introduced in 2018. The rotating bezel with a 24-hour scale in blue and burgundy gives it the colours of the classic GMT watches, but the execution is typically Tudor: a 41 mm case, snowflake hands and a black dial with a date window at three o'clock.
The in-house calibre MT5652, COSC-certified and with about 70 hours of power reserve, has an independently adjustable hour hand. On arrival in another time zone you move the local time forward hour by hour without disturbing the GMT hand or the minutes, and the date follows. With the rotating bezel you can read a third time zone if you wish. The watch is water-resistant to 200 metres and was supplied on a steel bracelet or on a strap.
On a pre-owned example we test the GMT function and the date, and check the condition of the aluminium bezel insert and its click when turned, the luminous material, the bracelet and clasp, and the set. Anyone looking for a more compact watch with two time zones will find it in the 39 mm Black Bay Pro.