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Aggregation Operators

$dateAdd (aggregation)

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  • Definition
  • Behavior
  • Examples

Definition

$dateAdd

New in version 5.0.

Increments a Date() object by a specified number of time units.

The $dateAdd expression has the following syntax:

{
   $dateAdd: {
      startDate: <Expression>,
      unit: <Expression>,
      amount: <Expression>,
      timezone: <tzExpression>
   }
}

Returns a Date(). The startDate can be any expression that resolves to type Date, Timestamp or ObjectId. No matter which data type is used as input, the value returned will be a Date() object.

Field
Required/Optional
Description
startDate
Required
The beginning date, in UTC, for the addition operation. The startDate can be any expression that resolves to a Date, a Timestamp, or an ObjectID.
unit
Required

The unit used to measure the amount of time added to the startDate. The unit is an expression that resolves to one of the following strings:

  • year

  • quarter

  • week

  • month

  • day

  • hour

  • minute

  • second

  • millisecond

amount
Required
The number of units added to the startDate. The amount is an expression that resolves to an integer or long. The amount can also resolve to an integral decimal or a double if that value can be converted to a long without loss of precision.
timezone
Optional

The timezone to carry out the operation. <tzExpression> must be a valid expression that resolves to a string formatted as either an Olson Timezone Identifier or a UTC Offset. If no timezone is provided, the result is displayed in UTC.

Format
Examples
Olson Timezone Identifier
"America/New_York"
"Europe/London"
"GMT"
UTC Offset
+/-[hh]:[mm], e.g. "+04:45"
+/-[hh][mm], e.g. "-0530"
+/-[hh], e.g. "+03"

For more information on expressions and types see Expression Operators and BSON Types.

Behavior

Time Measurement

MongoDB follows prevaling database usage and works with time in UTC. The dateAdd expression always takes a startDate in UTC and returns a result in UTC. If the timezone is specified, the calculation will be done using the specified timezone. The timezone is especially important when a calculation involves Daylight Savings Time (DST).

If the unit is a month, or larger the operation adjusts to account for the last day of the month. Adding one month on the last day of October, for example, demonstrates the "last-day-of-the-month" adjustment.

{
   $dateAdd:
      {
         startDate: ISODate("2020-10-31T12:10:05Z"),
         unit: "month",
         amount: 1
      }
}

Notice that the date returned, ISODate("2020-11-30T12:10:05Z"), is the 30th and not the 31st since November has fewer days than October.

Time Zone

When using an Olson Timezone Identifier in the <timezone> field, MongoDB applies the DST offset if applicable for the specified timezone.

For example, consider a sales collection with the following document:

{
   "_id" : 1,
   "item" : "abc",
   "price" : 20,
   "quantity" : 5,
   "date" : ISODate("2017-05-20T10:24:51.303Z")
}

The following aggregation illustrates how MongoDB handles the DST offset for the Olson Timezone Identifier. The example uses the $hour and $minute operators to return the corresponding portions of the date field:

db.sales.aggregate([
{
   $project: {
      "nycHour": {
         $hour: { date: "$date", timezone: "-05:00" }
       },
       "nycMinute": {
          $minute: { date: "$date", timezone: "-05:00" }
       },
       "gmtHour": {
          $hour: { date: "$date", timezone: "GMT" }
       },
       "gmtMinute": {
          $minute: { date: "$date", timezone: "GMT" } },
       "nycOlsonHour": {
          $hour: { date: "$date", timezone: "America/New_York" }
       },
       "nycOlsonMinute": {
          $minute: { date: "$date", timezone: "America/New_York" }
       }
   }
}])

The operation returns the following result:

{
   "_id": 1,
   "nycHour" : 5,
   "nycMinute" : 24,
   "gmtHour" : 10,
   "gmtMinute" : 24,
   "nycOlsonHour" : 6,
   "nycOlsonMinute" : 24
}

Examples

Add a Future Date

Consider a collection of customer orders with these documents:

db.shipping.insertMany(
  [
     { custId: 456, purchaseDate: ISODate("2020-12-31") },
     { custId: 457, purchaseDate: ISODate("2021-02-28") },
     { custId: 458, purchaseDate: ISODate("2021-02-26") }
  ]
)

The normal shipping time is 3 days. You can use $dateAdd in an aggregation pipeline to set an expectedDeliveryDate 3 days in the future.

db.shipping.aggregate(
   [
      {
         $project:
            {
               expectedDeliveryDate:
                  {
                     $dateAdd:
                        {
                           startDate: "$purchaseDate",
                           unit: "day",
                           amount: 3
                        }
                  }
            }
       },
       {
          $merge: "shipping"
       }
    ]
 )

After adding 3 days to the purchaseDate with $dateAdd in the $project stage, the $merge stage updates the original documents with the expectedDeliveryDate.

The resulting documents look like this:

{
   "_id" : ObjectId("603dd4b2044b995ad331c0b2"),
   "custId" : 456,
   "purchaseDate" : ISODate("2020-12-31T00:00:00Z"),
   "expectedDeliveryDate" : ISODate("2021-01-03T00:00:00Z")
}
{
   "_id" : ObjectId("603dd4b2044b995ad331c0b3"),
   "custId" : 457,
   "purchaseDate" : ISODate("2021-02-28T00:00:00Z"),
   "expectedDeliveryDate" : ISODate("2021-03-03T00:00:00Z")
}
{
    "_id" : ObjectId("603dd4b2044b995ad331c0b4"),
   "custId" : 458,
   "purchaseDate" : ISODate("2021-02-26T00:00:00Z"),
   "expectedDeliveryDate" : ISODate("2021-03-01T00:00:00Z")
}

Filter on a Date Range

Update the shipping collection from the last example with this code to add delivery dates to the documents:

db.shipping.updateOne(
   { custId: 456 },
   { $set: { deliveryDate: ISODate( "2021-01-10" ) } }
)
db.shipping.updateOne(
  { custId: 457 },
  { $set: { deliveryDate:  ISODate( "2021-03-01" ) } }
)
db.shipping.updateOne(
   { custId: 458 },
   { $set: { deliveryDate:  ISODate( "2021-03-02" ) } }
)

You want to find late shipments. Use $dateAdd in a $match stage to create a filter that matches documents in a range of dates defined by a starting point ($purchaseDate) and a time period given by $dateAdd.

db.shipping.aggregate(
   [
      {
         $match:
            {
               $expr:
                  {
                     $gt:
                        [ "$deliveryDate",
                          {
                             $dateAdd:
                                {
                                   startDate: "$purchaseDate",
                                   unit: "day",
                                   amount: 5
                                }
                           }
                        ]
                  }
            }
       },
       {
          $project:
             {
                _id: 0,
                custId: 1,
                purchased:
                   {
                       $dateToString:
                          {
                             format: "%Y-%m-%d",
                             date: "$purchaseDate"
                          }
                   },
                delivery:
                   {
                      $dateToString:
                         {
                            format: "%Y-%m-%d",
                            date: "$deliveryDate"
                         }
                   }
             }
       }
   ]
)

The $match stage uses $gt and $dateAdd in an expression ($expr) to compare the actual deliveryDate with an expected date. Documents with delivery dates more than 5 days after the purchaseDate are passed on to the $project stage.

The $project stage uses the $dateToString expression to convert the dates to a more readable format. Without the conversion, MongoDB returns the date in ISODate format and assumes a UTC timezone.

In this example only one record is returned:

{ "custId" : 456, "purchased" : "2020-12-31", "delivery" : "2021-01-10" }

Adjust for Daylight Savings Time

All dates are stored internally in UTC time. When a timezone is specified, $dateAdd uses local time to carry out the calculations. The results are displayed in UTC.

You have customers in several timezones and you want to see what effect daylight savings time might have on your billing periods if you bill by day or by hour.

Create this collection of connection times:

db.billing.insertMany(
   [
      {
         location: "America/New_York",
         login: ISODate("2021-03-13T10:00:00-0500"),
         logout: ISODate("2021-03-14T18:00:00-0500")
      },
      {
         location: "America/Mexico_City",
         login: ISODate("2021-03-13T10:00:00-00:00"),
         logout: ISODate("2021-03-14T08:00:00-0500")
      }
   ]
)

First add 1 day, then add 24 hours to the login dates in each document.

db.billing.aggregate(
   [
      {
         $project:
            {
               _id: 0,
               location: 1,
               start:
                  {
                     $dateToString:
                        {
                           format: "%Y-%m-%d %H:%M",
                           date: "$login"
                        }
                  },
               days:
                  {
                     $dateToString:
                        {
                           format: "%Y-%m-%d %H:%M",
                           date:
                              {
                                 $dateAdd:
                                    {
                                       startDate: "$login",
                                       unit: "day",
                                       amount: 1,
                                       timezone: "$location"
                                    }
                              }
                        }
                  },
               hours:
                  {
                     $dateToString:
                        {
                           format: "%Y-%m-%d %H:%M",
                           date:
                              {
                                 $dateAdd:
                                 {
                                    startDate: "$login",
                                    unit: "hour",
                                    amount: 24,
                                    timezone: "$location"
                                 }
                              }
                        }
                  },
               startTZInfo:
                  {
                     $dateToString:
                        {
                           format: "%Y-%m-%d %H:%M",
                           date: "$login",
                           timezone: "$location"
                        }
                  },
               daysTZInfo:
                  {
                     $dateToString:
                        {
                           format: "%Y-%m-%d %H:%M",
                           date:
                              {
                                 $dateAdd:
                                    {
                                       startDate: "$login",
                                       unit: "day",
                                       amount: 1,
                                       timezone: "$location"
                                    }
                              },
                           timezone: "$location"
                        }
                  },
               hoursTZInfo:
                  {
                     $dateToString:
                        {
                           format: "%Y-%m-%d %H:%M",
                           date:
                              {
                                 $dateAdd:
                                    {
                                       startDate: "$login",
                                       unit: "hour",
                                       amount: 24,
                                       timezone: "$location"
                                    }
                              },
                           timezone: "$location"
                        }
                  },
            }
      }
   ]
).pretty()

The $dateToString expression reformats the output for readability. Results are summarized here:

Field
New York
Mexico City
Start
2021-03-13 15:00
2021-03-13 10:00
Start, TZ Info
2021-03-13 10:00
2021-03-13 04:00
1 Day
2021-03-14 14:00
2021-03-14 10:00
1 Day, TZ Info
2021-03-14 10:00
2021-03-14 04:00
24 Hours
2021-03-14 15:00
2021-03-14 10:00
24 Hours, TZ Info
2021-03-14 11:00
2021-03-14 04:00

The chart highlights several points:

  • Unformatted dates are returned in UTC. The $login for New York is UTC -5, however the start, days, and hours rows display the time in UTC.

  • March 14th is the start of DST in New York, but not in Mexico. The calculated time is adjusted when a location switches to DST and crosses from one day to the next.

  • DST modifies the length of the day, not the hour. There is no DST change for hours. There is an only an adjustment for DST when the measurement unit is day or larger and the computation crosses a clock change in the specified timezone.

Tip

See also:

$dateSubtract, $dateDiff

Back

$covarianceSamp (aggregation)