The makers of a new smartwatch claim it can go for up to 30 days without a charge. The key to the TicWatch Pro’s durability is a dual-layer display, though there’s a big catch with the claim.

The $250 watch has all the gizmos and specs you’d expect including multiple motion and fitness sensors, 4GB of on-board storage and Bluetooth, NFC and Wi-Fi connections. It runs Google’s Wear operating system and includes the Assistant tool.

Its main display is a 400×400 circular color OLED screen similar to that used on many smartwatches. However, on top of that lies a transparent monochrome LCD screen. It works with FSTN (Film Compensated Super Twist Nematic) fluid, a similar set-up to that in older laptop screens. By setting this screen to an ‘empty’ display, the watch can show the OLED screen beneath without obstruction.

To balance functionality and battery life, most of the time the watch will be using the LCD screen in ‘Essential’ mode, thus using a similar amount of power for the display as an ordinary digital watch. The essential mode will still display the time and date plus stats for heartrate and step counts.

If you need to do anything else, the watch switches to ‘Smart’ mode which turns on the OLED screen and makes all data and functions available. This switch also triggers the watch to sync activity data.

Only time will tell how well this actually works in practice. While the 30 day stat is a key part of the marketing, this is only possible if the watch is permanently in Essential mode, at which point it’s not really a very smart phone. Using the automatic switching between modes the claim is only “a minimum battery life of 5 days.”