"improve this script and tell me how u did improve it and also unknown global b and t"

The script is controlling the state of two lights based on the time of day. The lights are turned on and off at specific times. However, the script could be improved in several ways:

1. Use descriptive variable names: Instead of using single letter variable names like 'b', 't', 'l', 'oh', 'om', 'ch', 'cm', 'ot', and 'ct', use descriptive names that explain what the variables represent. This makes the script easier to understand and maintain.

2. Handle nil values: The script assumes that the SpotLight and Light exist. It would be better to handle the case where they do not exist.

3. Use constants for magic numbers: The numbers 7, 30, 17, and 30 are used without explanation. It would be better to define these as constants at the top of the script, with names that explain what they represent.

4. Remove unnecessary if statements: The script checks if 'om' and 'cm' are nil, but these are hard-coded values and will never be nil.

Here's how the script could look after these improvements:

```lua
-- Constants
local OPEN_HOUR = 7
local OPEN_MINUTE = 30
local CLOSE_HOUR = 17
local CLOSE_MINUTE = 30

-- Ensure the necessary parts exist
assert(script.Parent:FindFirstChild("SpotLight"), "SpotLight not found")
assert(script.Parent:FindFirstChild("Light"), "Light not found")

local spotLight = script.Parent.SpotLight
local light = script.Parent.Light

local lightingService = game:GetService("Lighting")

-- Convert hours and minutes to minutes past midnight
local openTime = (OPEN_HOUR + (OPEN_MINUTE / 60)) * 60
local closeTime = (CLOSE_HOUR + (CLOSE_MINUTE / 60)) * 60

local function TimeChanged()
    local currentTime = lightingService:GetMinutesAfterMidnight()

if (currentTime >= openTime and currentTime <= closeTime) or
       (currentTime >= openTime or currentTime <= closeTime) then
        spotLight.Enabled = false
        light.Enabled = false
    else
        spotLight.Enabled = true
        light.Enabled = true
    end
end

TimeChanged()

lightingService.Changed:Connect(function(property)
    if property == "TimeOfDay" then
        TimeChanged()
    end
end)
```

In this improved version, the script is more readable and robust. It handles the case where the necessary parts do not exist, uses descriptive variable names, and removes unnecessary checks for nil values. It also uses constants for magic numbers, making the script easier to understand and modify.