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I'm new to MongoDB and my requirement is to convert a string date to date. But that particular field is sometimes in date format sometimes in string format.

Effectively, If the date is in string format I want to convert it to date else leave as it is.

Sample data:

paymentDate:2021-11-19T05:36:32.596+00:00

paymentDate:'2021-11-19T05:36:32.596+00:00'

My attempt is

convertedDate: { $cond: { {'$eq': [{$type:"$paymentDate"},9]}, then:"$newField", else:{ $dateFromString: { dateString: '$paymentDate'

You're almost to the answer. Specify the compare value in $type as "date".

db.collection.find({},
  convertedDate: {
    $cond: {
      if: {
        "$eq": [
            $type: "$paymentDate"
          "date"
      then: "$paymentDate",
      else: {
        $dateFromString: {
          dateString: "$paymentDate"
Sample Mongo Playground
References
Available type | $type
                Hi @Julien, I rollback your edit as my answer is for query but not update. Anyway thanks for the efforts.
– Yong Shun
                Nov 19, 2021 at 9:06
                Post Owner may take note that $dateFromString can be replaced with { '$toDate': '$paymentDate' } as mentioned by @ray.
– Yong Shun
                Nov 19, 2021 at 9:08
                Because 9 is not a valid type, check out the reference link I shared to you for list of types in MongoDB.
– Yong Shun
                Nov 19, 2021 at 11:06
If you are using MongoDB 4.2+, you may simply use $toDate to convert your field in an update with aggregation pipeline operation.
db.collection.update({},
    "$set": {
      "paymentDate": {
        "$toDate": "$paymentDate"
Here is the Mongo playground for your reference.
        
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