Law departments might consider several points in time to convert invoices that are not delivered in their home currency (See my posts of July 17, 2005 on currency considerations.). The date of the invoice might be when the law department ought to convert. The date might be later, such as when the law department enters the invoice into its matter management system (See my post of Sept. 5, 2005 on conversion on that date.). Or, the law department might take the currency conversion date of its accounts payable system, whatever that is.
Regardless of the date chosen, the law department also has to decide on the conversion figure to use. One department I worked with selected a conversion figure and held it constant for the entire year. Others update quarterly, monthly or even daily. Online systems can maintain very fresh conversion dates.