# How is -%{+YYYY.MM.dd} generated on Logstash output

**URL:** <https://discuss.elastic.co/t/how-is-yyyy-mm-dd-generated-on-logstash-output/142189>\
**Category:** Logstash\
**Created:** [July 30, 2018, 1:52pm UTC](https://discuss.elastic.co/t/how-is-yyyy-mm-dd-generated-on-logstash-output/142189 "2018-07-30T13:52:40Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![A\_B](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/a_b/32/17104_2.png) [@A\_B](https://discuss.elastic.co/u/A_B)\
**Post date:** [July 30, 2018, 1:52pm UTC](https://discuss.elastic.co/t/how-is-yyyy-mm-dd-generated-on-logstash-output/142189/1 "2018-07-30T13:52:40Z")

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Hello,

I use daily ES indices that are created by Logstash using

```
output {

  elasticsearch {
...
        index => "%{[@metadata][foo]}-%{+YYYY.MM.dd}"
  }
}

```

I always assumed that Logstash looks up the date from the system but now I am seeing some evidence that it might come from @timestamp in the log message. Could someone confirm this suspicion?

Cheers,  
AB

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<div class="post-metadata">

**Author:** ![Badger](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/badger/32/25190_2.png) [@Badger](https://discuss.elastic.co/u/Badger)\
**Post date:** [July 30, 2018, 2:25pm UTC](https://discuss.elastic.co/t/how-is-yyyy-mm-dd-generated-on-logstash-output/142189/2 "2018-07-30T14:25:22Z")

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Confirmed. It is based on @timestamp.

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<div class="post-metadata">

**Author:** ![A\_B](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/a_b/32/17104_2.png) [@A\_B](https://discuss.elastic.co/u/A_B)\
**Post date:** [July 30, 2018, 2:48pm UTC](https://discuss.elastic.co/t/how-is-yyyy-mm-dd-generated-on-logstash-output/142189/3 "2018-07-30T14:48:01Z")

</div>

Thank you @Badger

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<div class="post-metadata">

**Author:** ![magnusbaeck](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/magnusbaeck/32/44943_2.png) [@magnusbaeck](https://discuss.elastic.co/u/magnusbaeck)\
**Post date:** [August 1, 2018, 8:35pm UTC](https://discuss.elastic.co/t/how-is-yyyy-mm-dd-generated-on-logstash-output/142189/4 "2018-08-01T20:35:52Z")

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The behavior is documented here: [https://www.elastic.co/guide/en/logstash/current/event-dependent-configuration.html#sprintf](https://www.elastic.co/guide/en/logstash/current/event-dependent-configuration.html#sprintf)

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<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\
**Post date:** [August 29, 2018, 8:35pm UTC](https://discuss.elastic.co/t/how-is-yyyy-mm-dd-generated-on-logstash-output/142189/5 "2018-08-29T20:35:54Z")

</div>

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