Transaction auxiliary date.

[prataprc]

You can associate a second date with a transaction by following the primary date with an equals sign:

2012-03-10=2012-03-08 KFC
    Expenses:Food             $20.00
    Assets:Cash                 $-20.00

What this auxiliary date means is entirely up to you. The only use Ledger has for it is that if you specify --aux-date, then all reports and calculations (including pricing) will use the auxiliary date as if it were the primary date.