Group Join Error Message

[ptssmith]

When I try to join a vessel to a group, I sometimes get the following error message;

"Unit's date of commissioning is after leader's entry date"

After looking at everything I can think of, I still have no idea what the error message means. The terms 'commissioning' and 'entry' are giving me no help / hints / help.
Does anyone have an idea what is being addressed with this error message?

[MWolfe1963]

Simply means the ship you're trying to join to the group hasn't been made or comissioned yet.
Whatever ship you create the group from, the leader, other added ships dates comissioned must come before it...make sense.
Pick other ships to add made before or if you want a ship, pick a leader made after it..