Java Edition:Classic Level Creation Fields
The createTime, creator, and name fields were added to save files in Classic 0.0.13a-dev. These fields are normally initialized when a level is created, however, if a level created before these fields existed is updated to Classic 0.0.13a-dev or later, the fields will be initialized with discontinued values. These values are unobservable without the use of external tools, and is a file discontinued feature.
Obtaining
To obtain this discontinued feature, a world must be created in a version between rd-132211 and Classic 0.0.12a_03 then this world must be updated to a version between Classic 0.0.13a-dev and Classic 0.0.23a_01 to which these three fields will be set to discontinued values. Attempting to load this world straight to Classic 0.0.23a_01 or later will not work and must be loaded in an earlier version. This discontinued feature is mutually exclusive with the Noname Creator discontinued feature.
Field Values
createTime
The "createTime" field is usually initialized with the Unix timestamp of the worlds creation, but when updating, this field will be initialized as 0. The 0 Unix timestamp represents January 1st, 1970 00:00:00 (UTC) as the Unix timestamp is the amount of seconds since that date, this creates a "createTime" value that should be impossible as Minecraft did not exist at that time meaning there is no legitimate method to obtain this other then manipulating the systems clock.
creator
The "creator" field is usually initialized with the username of the player who created the world, but when updating, this field will be initialized as "unknown" instead. This is not a discontinued feature for the player named "unknown" but as the username "Unknown" is currently claimed the "unknown" value is impossible to obtain unless the "Unknown" user were to change their name or delete their account.[1]
name
The "name" field is usually initialized with "A Nice World", but when updating, this field will be initialized as "--" (two dashes) instead. This is not the worlds name as seen in-game, but is the name of the world file.
Observability
Because this feature is a file discontinued feature, it is unobservable in-game. The only way to view this discontinued feature is by using external tools, both ClassicExplorer and NBTExplorer are external tools which can be used to view and manipulate these fields.