Calendar year starts with January 1 and ends with December 31.
For instance, Calendar year 2019 starts with 1/1/2019 and ends with 12/31/2019.
Fiscal year is a period that a company or government uses for taxing or accounting purposes. It does NOT have to start with January 1 and it varies among companies.
For instance, the U.S. government’s fiscal year starts from October 1 and ends with September 31.
A company may find benchmarking its performance to the performance of its peers more accurate if it's using the same fiscal year.
How to decide the fiscal year?
Fiscal year is decided based on fiscal year-end date: For firms with a fiscal year-end date between January and May, the fiscal year lags one year behind the calendar year. For firms with a fiscal year-end month between June and December, the fiscal year is same as the calendar year.
How to find the fiscal year-end date?
Fiscal year-end date can be found from the company information section on the top of the filing folder (inside the red circle below).
Typically, 10-K is filed within 4 months after the fiscal year-end date. For instance, if a company’s fiscal year-end date is 12/31, then for the 10-K of fiscal year 2018, the filing date is normally between 1/1/2019 and 4/30/2019.
The filing date for each filing can be found in company’s filing folder (inside the red circle below)
For instance, the fiscal year-end date of Tesla, Inc for fiscal year 2018 should be 2018-12-31. Then, the 10-K filed on 2019-02-19 (1st filings in the table above) should be the annual report for fiscal year 2018.