Hola Mayela
Ok, so 2017 is setup correctly Jan 1 2017 to Dec 31, 2017 but 2018 is showing Jan 1 2019 to Dec 31 2019
Here is what you should try.
First have everyone exit your Dynamics GP company and make a backup of the database.
Next in SQL Management Studio, run the following script against the company database
Select * from SY40101
You will see each year and the start and end date. Note the DEX_ROW_ID for the year 2018 with the start date of Jan 1 2019 and the end date of Dec 31 2019
Run the following
Delete SY40101 where DEX_ROW_ID = 'xxx'
where xxx = the dex row number you found in the select script
Next you will need to run the following script
Select * from SY40100 where PERIODDT > '2017-12-31' order by PERIODID
You should see quite a few records returned.
View the results and ensure that the dates are indeed all for 2019.
If they are then you can run the following script:
Delete SY40100 where PERIODDT > '2017-12-31'
Next you can go back into the fiscal calendar and enter 2018 for the new fiscal. The first day should be Jan 1 2018 and the last day should be December 31, 2018. If not change them to the correct dates.
Click on Calculate.
After you do this I recommend running the GL reconcile on all your years just in case.