How do you calculate LTM (last twelve months) financials?

LTM = New partial period - Old partial period + Most recent full fiscal year

For example, let's assume a company has a December 31 fiscal year end, we want to calculate LTM revenue, and we are currently in the month of April 2021.

First we will take revenue from Q1 2021.

Then we will subtract Q1 2020 revenue.

Finally we will add FY 2020 revenue to get LTM Q1 revenue.