The $dayOfMonth operator extracts the day number (1–31) from a BSON Date inside MongoDB aggregation pipelines. Use it for billing cycles, first-of-month reports, birthday-day matching, or any query that depends on which day of the month an event occurred.
Date → integer 1–31.
One date argument.
Integer, not Date.
Uses UTC calendar.
Billing, birthdays.
$month, $dayOfWeek.
In MongoDB’s aggregation framework, the $dayOfMonth operator takes a date expression and returns an integer between 1 and 31. For example, $dayOfMonth applied to ISODate("2024-06-15T14:30:00Z") returns 15. The result is always a number, not a Date or string.
$dayOfMonth uses the UTC calendar. A timestamp near midnight in your local zone may show a different day than you expect — use $dateToParts with timezone when local calendar days matter.
The $dayOfMonth operator takes a single date expression:
{ $dayOfMonth: <dateExpression> }Date (field, $$NOW, or literal).null when the date is null or missing.$project, $addFields, $set, $match (with aggregation syntax), or $group.$month and $year for full calendar filtering.| Question | Answer |
|---|---|
| Operator type | Aggregation expression operator (date extraction) |
| Arguments | One date expression |
| Return type | Integer (1–31) |
| Timezone | UTC (use $dateToParts for local) |
| MongoDB version | 3.4+ |
{ $dayOfMonth: "$createdAt" }Day of createdAt
{ $dayOfMonth: ISODate("2024-06-15") }Returns 15
{
billingDay: {
$dayOfMonth: "$renewalDate"
}
}Add computed field
{
m: { $month: "$d" },
dom: { $dayOfMonth: "$d" }
}Month + day pair
Extract billing days, filter first-of-month events, match birthday days, and analyze activity patterns with $dayOfMonth.
Use a subscriptions collection and read the renewal day from renewalDate.
Suppose you have a subscriptions collection with renewal dates:
[
{
"_id": ObjectId("609c26812e9274a86871bc6a"),
"plan": "Pro",
"renewalDate": ISODate("2024-06-15T00:00:00Z")
},
{
"_id": ObjectId("609c26812e9274a86871bc6b"),
"plan": "Basic",
"renewalDate": ISODate("2024-03-01T12:00:00Z")
}
]Extract the day number from each subscription’s renewal date:
db.subscriptions.aggregate([
{
$project: {
plan: 1,
renewalDate: 1,
billingDay: { $dayOfMonth: "$renewalDate" }
}
}
])15; March 1 → 1.billingDay if you query by it often.First-of-month filters, birthday matching, and grouping by day of month.
Filter subscriptions that renew on day 1 using $expr and $dayOfMonth:
db.subscriptions.find({
$expr: {
$eq: [
{ $dayOfMonth: "$renewalDate" },
1
]
}
})
// Returns documents where renewalDate falls on UTC day 1In a standard find query, wrap aggregation expressions in $expr. This pattern works for any day number — replace 1 with 15 for mid-month billing, for example.
Combine $dayOfMonth with $month to find users with birthdays today:
db.users.aggregate([
{
$match: {
$expr: {
$and: [
{ $eq: [ { $dayOfMonth: "$birthDate" }, { $dayOfMonth: "$$NOW" } ] },
{ $eq: [ { $month: "$birthDate" }, { $month: "$$NOW" } ] }
]
}
}
},
{
$project: {
name: 1,
birthDate: 1
}
}
])Matching on month and day ignores the birth year, so you can send birthday emails or promotions. For local-time birthdays, extract parts with $dateToParts and a timezone instead.
Analyze whether orders cluster on certain days (e.g. paydays):
db.orders.aggregate([
{
$group: {
_id: { $dayOfMonth: "$orderDate" },
orderCount: { $sum: 1 },
totalAmount: { $sum: "$amount" }
}
},
{ $sort: { _id: 1 } }
])Using $dayOfMonth directly in $group._id buckets all orders from the 1st across every month into one group. Combine with $month or $year in _id when you need finer breakdowns.
$dayOfMonth WorksThe input expression is evaluated per document — a field, $$NOW, or literal ISODate.
MongoDB determines which day of the month the instant falls on in UTC.
The value 1–31 is available for filtering, grouping, or display alongside other date parts.
Combine with $month, $year, or $dayOfWeek for rich calendar logic in pipelines.
The $dayOfMonth operator is a simple, fast way to pull the day number from BSON dates in MongoDB aggregation pipelines. One expression replaces manual date parsing for billing days, calendar filters, and day-of-month analytics.
Remember it uses UTC. For timezone-aware extraction, use $dateToParts. For full period bucketing, prefer $dateTrunc. Next in the series: $dayOfWeek for weekday extraction.
$month for birthday or anniversary matching$dateToParts + timezone for local calendar days$expr in find() when you need $dayOfMonth$year to $group._id when you need per-month buckets$dayOfMonth with $dayOfWeek (1–7 weekday)$dateFromString first$dayOfMonthUse these points when extracting the day from dates in MongoDB.
Date → 1–31.
PurposeDate expression.
SyntaxNot a Date.
OutputDefault timezone.
NoteFull date parts.
ImportantMove on to $dayOfWeek to extract weekday numbers, or review $dateToParts for timezone-aware components.
5 people found this page helpful