Hi @Danilo,
Please share the structure of the custom field in question. The JavaScript GetHours() method can return the hours from a date - Date.prototype.getHours() - JavaScript | MDN.
You may want to transform the custom field value to date first with new Date(Date.parse(issue.fields.customfield_13227)). You can see such an example for retrieving the hours of issue creation from the creation date here - Generate count of issues created per day and per hour.
You can access the custom field structure in custom JavaScript code in the eazyBI import options. See more details here - JavaScript calculated custom fields.
Best,
Roberts // support@eazybi.com