$dayOfYear (Get Day of Year)

Last updated: 2026-10-09

$dayOfYear (Get Day of Year)

$dayOfYear returns the day of the year of a date (1 to 366) — useful for yearly progress, seasonal analysis, or days-remaining calculations. Counting starts at 1; a common year tops out at 365, a leap year at 366. It computes in UTC or the given timezone, and the boundary shifts across zones, so pass timezone explicitly. Pair with the %j placeholder of $dateToString for a zero-padded three-digit string.

📝 Syntax

{ $dayOfYear: { date: <date-expression>, timezone: <timezone> } }

⚙️ Parameters

Returns：Day of year (1-366) as an integer; null if date is null.

▶ Example

Run the commands below in your local mongosh (requires a running mongod):

// $dayOfYear: day-of-year of events
 db.events.aggregate([
  { $project: {
      doy: { $dayOfYear: '$eventDate' },
      label: { $dateToString: { format: '%j', date: '$eventDate' } }
  } }
])

Output：

{ "_id": 1, "doy": 268, "label": "268" }
{ "_id": 2, "doy": 1, "label": "001" }

📌 Notes

• Jan 1 is 1; Dec 31 is 365 in a common year, 366 in a leap year
• Use %j for a zero-padded three-digit string
• Boundaries follow UTC/timezone like $week — unify zones before stats

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