The US tax code requires you to maintain contemporaneous records to substantiate several types of deductions, including vehicle mileage and charitable contributions of $250 or more.
For charitable contributions, a “contemporaneous written acknowledgment” means you obtain the acknowledgment on or before the earlier of the date on which you file your return for the taxable year in which the contribution was made, or the due date (including extensions) for filing such return. (Section 170(f)(8)(C)).
The definition of “contemporaneous” is a bit different for vehicle mileage. Per Temporary Regulation 1.274-5, a contemporaneous log is not required. However, the regulation also states that corroborative evidence of your vehicle use must be made “at or near the time of the use.” That phrase means the record is made when “in relation to the use or making of an expenditure, the taxpayer has full present knowledge of each element of the expenditure or use, such as the amount, time, place, and business purpose of the expenditure and business relationship”. (Temp. Reg. 1.274-5T (B). According to the regulation, a log maintained on a weekly basis is considered a record made at or near the time of use.