What if I forgot to log a shift?

Add it with the date it actually happened. There is no penalty for logging late and nothing about the calculation depends on when you typed it in.

Rates are stored per shift as a snapshot, so a shift you backdate into an earlier tax year uses that year's mileage rates and your settings as they were, not today's. That is what makes a corrected record trustworthy.

Reconstructing from a platform statement is fine, and it is what most people end up doing for a week they were too busy to log. Earnings come off the statement; the miles usually have to come off your odometer or a route history.

If you cannot recover the miles, it is better to leave them out than to invent them. An underclaim is a shame; a figure you cannot support is a problem.