It was alluded to earlier that these solar watches can be indeed charged without access to the sun. To accomplish this, you need to replace sunlight with artificial light. This type of light is merely any light source that is produced via electricity. So, good examples of it would be halogen lamps, fluorescent lights, incandescent bulbs, and even LEDs. Once you find an optimal source of artificial light, you will want to keep your watch around 20 inches away from it.
As mentioned, though, this method is not nearly as efficient as sunlight. If you really wanted to fully charge your device via artificial light, it could take up to 150 hours. It is more suited to get a charge if there has not been sunlight for weeks and you know the battery is getting low.