# Basic Math in Logstash
**URL:** https://discuss.elastic.co/t/basic-math-in-logstash/206452
**Category:** Logstash
**Created:** [November 4, 2019, 5:26pm UTC](https://discuss.elastic.co/t/basic-math-in-logstash/206452 "2019-11-04T17:26:55Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Jim\_Thunder](https://discuss.elastic.co/u/Jim_Thunder)
#### Post date: [November 4, 2019, 5:26pm UTC](https://discuss.elastic.co/t/basic-math-in-logstash/206452/1 "2019-11-04T17:26:55Z")
All I want to do it subtract 6 from the "hour\_of\_day" field, because it's six hours ahead. Which from my understanding, it shouldn't be ahead b/c I used ruby. But whatever the case, it's wrong.
Here's my code:
```
ruby {
code => "event.set('[hour_of_day]',(event.get('@timestamp').time.strftime('%H') - 6))"
}
```
I get error messages on the `-` b/c it (ruby?) doesn't know what `-` is.
How can I fix that?
Much appreciated!
---
### Author:  [@Badger](https://discuss.elastic.co/u/Badger)
#### Post date: [November 4, 2019, 6:11pm UTC](https://discuss.elastic.co/t/basic-math-in-logstash/206452/2 "2019-11-04T18:11:48Z")
strftime returns a string. You are effectively trying to do
```
"12" - 6
```
It will not coerce "12" into 12, so you need to use .to\_i
```
ruby { code => "event.set('[hour_of_day]', (event.get('@timestamp').time.strftime('%H').to_i - 6))" }
```
How is @timestamp getting set? If it is set by a date filter then you can use the timezone option to adjust the hour.
---
### Author:  [@Jim\_Thunder](https://discuss.elastic.co/u/Jim_Thunder)
#### Post date: [November 4, 2019, 6:14pm UTC](https://discuss.elastic.co/t/basic-math-in-logstash/206452/3 "2019-11-04T18:14:12Z")
That worked. Got my time set to the correct hour.
But yea I use a date filter to set @timestamp. I didn't know I could change the timezone!
Point me in the right direction for that?
---
### Author:  [@Badger](https://discuss.elastic.co/u/Badger)
#### Post date: [November 4, 2019, 6:52pm UTC](https://discuss.elastic.co/t/basic-math-in-logstash/206452/4 "2019-11-04T18:52:55Z")
Use the [timezone](https://www.elastic.co/guide/en/logstash/current/plugins-filters-date.html#plugins-filters-date-timezone) option. Something like
```
date {
match => [...]
timezone => "Etc/GMT-6"
}
```
---
### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [December 2, 2019, 6:52pm UTC](https://discuss.elastic.co/t/basic-math-in-logstash/206452/5 "2019-12-02T18:52:55Z")
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