Each zettel is given a unique identifier. To some degree, the zettel identifier is part of the metadata. Basically, the identifier is given by the Zettelstore software.
Every zettel identifier consists of 14 digits. They resemble a timestamp: the first four digits could represent the year, the next two represent the month, following by day, hour, minute, and second.
This allows to order zettel chronologically in a canonical way.
In most cases the zettel identifier is the timestamp when the zettel was created.
However, the Zettelstore software just checks for exactly 14 digits. Anybody is free to assign a non-timestamp identifier to a zettel, e.g. with a month part of 35 or with 99 as the last two digits.
Some zettel identifier are reserved and should not be used otherwise. All identifiers of zettel initially provided by an empty Zettelstore begin with 000000, except the home zettel 00010000000000. Zettel identifier of this manual have be chosen to begin with 000010.
A zettel can have any identifier that contains 14 digits and that is not in use by another zettel managed by the same Zettelstore.