9:00 pm CT might be unsuitable for MST time zone
When planning a call between Central Time and Mountain Standard Time, you need to consider time difference between these time zones. CT is 2 hours ahead of MST.
It is 9:00 pm in CT. This does not fall within the span of usual working time between 9:00 am and 4:00 pm in MST, but it still might be suitable to arrange a meeting, since it would be 7:00 pm in MST, which is between 7:00 am and 11:00 pm MST time.