It is possible that Yahweh's people can keep appointments with heathens, using their time.

It is NOT possible for us to keep our appointments with Yahweh unless we use (obey, follow, observe) His time. The most important appointment we have with Yahweh is His Sabbath, which He tells us to remember. Sabbath is the 7th day. The 7th day is NOT Friday, NOT Saturday and NOT Sunday. These days didn't exist until the Roman beast of Satan made them. On the calendar that existed from the time of creation, Sabbath, the 7th day of the week, fell on the 8th, 15th, 22nd and 29th day of EVERY one of Yahweh's months. The 1st day of each month was treated like a Sabbath, but was actually called a "new moon" day.

A new week ALWAYS started the day after the new moon day. These days may fall on any named day of Satan's time and vary from month to month. (actually, Yahweh's time doesn't vary, Satan's does).

Every month in Yahweh's time is consistent, as the following calendar shows. The 7th day Sabbath ALWAYS falls on the 8th, 15th, 22nd and 29th days of each and every month. Study your Bible and you will verify this.