JCMT operates a 12-hour shift from approximately sunset to just after sunrise (though submillimeter observations are not strictly limited to nighttime hours). We have an attending Telescope System Specialist (TSS) who operates the telescope from the summit, and who is accompanied by a visiting observer. The visiting observer assists by helping to guide selection of MSBs with the Query Tool (according to weather and priority) and providing scientific feedback on the quality of the observations being collected. If weather conditions are favourable, we might also complete evening observing and then handover to a remote operator (Extended Observer) in Hilo, to continue observing for a few hours in the morning whilst conditions are still good.
Yes it can. Submillimeter telescopes are not limited to night time hours. Telescopes like the JCMT are mostly restricted by the amount of water vapour present in the atmosphere. This tends to be lower at night, and in the early morning, but can often (especially on sites like Mauna Kea) be low enough throughout the day to allow us to observe at any time. Scientific observations of planets and comets are the most common targets for daytime observing.
However usually the seeing becomes bad at submillimeter wavelengths during the day and pointing and focus observations become difficult after about 10am HST.