How do I make a TextLabel display the current UTC time with AM/PM?

For Lua 5.1, you can perform a simple conditional and a mathematical operation to calculate the 12-hour time and add the suffix:

local function formatTime(t)
	local dateTable = os.date("!*t", t)
	local hour, min, sec = dateTable.hour, dateTable.min, dateTable.sec
	local period = "AM"
		
	if hour >= 12 then
		period = "PM"
		hour = hour ~= 12 and hour - 12 or hour
	end
	
	return string.format("%.2d:%.2d:%.2d %s", hour, min, sec, period)
end

while true do
	print(formatTime())
	wait()
end

For newer Lua versions:

os.date accepts a format string. All the tags can be found in PIL. There are options for the 12 hour clock.

print(os.date("%I:%m:%S %p", os.time()))