6:00 pm MST might be unsuitable for ET time zone
When planning a call between Mountain Standard Time and Eastern Time, you need to consider time difference between these time zones. MST is 3 hours behind of ET.
It is 6:00 pm in MST. This does not fall within the span of usual working time between 12:00 pm and 6:00 pm in ET, but it still might be suitable to arrange a meeting, since it would be 9:00 pm in ET, which is between 7:00 am and 11:00 pm ET time.