ObjectId, Date and Timestamp

Every document you insert without an _id gets an ObjectId, and every "when did this happen?" question needs a Date. They look alike in the shell, both mention time, and a third type called Timestamp muddies it further. One rule sorts them out.

The rule

ObjectId identifies, Date records when, Timestamp is MongoDB's own business.

ObjectId is a 12-byte value, shown as 24 hexadecimal characters:

• The first 4 bytes are a timestamp in seconds, so ObjectId(...).getTimestamp() returns when it was generated.
• The next 5 bytes are random per process, and the last 3 bytes are a counter.
• It is unique in practice without asking the server, which is why drivers can create it themselves. Sorting by _id gives roughly creation order, but only to the second.

Date is a signed 64-bit count of milliseconds since 1 January 1970, UTC. Use it for every application time: order placed, user joined, book released.

mongosh shows Dates as ISODate("..."), always in UTC. Convert to local time in your app, not in the database.

Timestamp is a separate BSON type: 32 bits of seconds plus a 32-bit counter. MongoDB uses it internally, for example in the replication log. The official advice is not to use it in your own documents; use Date.

The shortcut

Ask "Am I naming a document or recording a moment?" Naming → ObjectId. Moment → Date. Timestamp → almost never.

The trap: placedAt: Date() without new. In JavaScript that returns a string, so the field is text, not a Date. Always write new Date() or ISODate(...).

Drills

Answer fast — trust the rule.

Remember

• ObjectId identifies, Date records when, Timestamp is MongoDB's own business.
• Shortcut: naming a document → ObjectId; recording a moment → new Date().
• Trap: Date() without new stores a string, not a date.