The tax law has strict rules for supporting travel expenses. You need to be able to document your travel with records showing the date, amounts, and business purpose. For mileage, the tax law likes a contemporaneous diary.
The IRS audited the travel expenses of a Florida real estate agent. They disallowed about $10,000 in vehicle expenses, and it ended up in Tax Court, where things went badly in a tax nerd Perry Mason moment:
Under cross-examination by respondent’s counsel, Ms. [real estate agent] acknowledged that some of the entries in the notebook had been altered (i.e., the portion of the date indicating the year was obliterated) and that one of the entries is for a date in 2010. In addition, the day planner included an order form which provided a convenient way for the owner to purchase a new day planner for the coming year. In this case, the order form was for the calendar year 2014, a fact that completely undermined Ms. [real estate agent]‘s testimony that she recorded information in the day planner contemporaneously in 2008.
Oops. The IRS won, with penalties.
The moral: There are a lot of ways the IRS can trip you up if you try to cook up your mileage diary retroactively. If you really want the deduction, record your travel as you go, either on an old-fashioned auto log or one of those smartphone mileage apps.
Thanks to Lileks for the Perry Head.