I want to get connectionTime of all activePlayerList. VisualStudio displays that it should be common DateTime and not showing any error. If I copy the plugin to the server then I get:
Error while compiling: cannot implicitly convert type 'double' to 'System.DateTime'

I tried to 'Puts' the real value of player.Connection.connectionTime (example below) and it really shows double (89911.0625)

Any idea?

foreach (BasePlayer player in BasePlayer.activePlayerList)
            {
                Puts(player.Connection.connectionTime.ToString());
            }

Hm... 

I see two reasons for behaviour like you've described:

1. In the dll you've referenced there is different type for this field (outdated dll, however I doubt that field have been changed)
2. Intellisense bug (happens sometimes, especially after IntelliCode introduction in VS2019, fixed by IDE restart)

Yes, they changed it in Facepunch.Network.dll. I copied the fresh files from the server and now I see double as well.

But what is the point? I would like to see the datetime of connection! :) So I will use GetSecondsConnected to calculate the time.

Thank you very much. At least you make me install VS2019 (from 2017)