I would like to know, say, what 1$ on January 1st, 2010 is worth in today's €.
I'm no economist, but that probably means:
- Applying "the" $-€ conversion rate that was in place on that day, obtaining January 1st, 2010 euros
- Adjusting that amount for inflation to convert it into euros of today's.
Something that ran offline would be preferable (any platform but Mac) to something that ran online with APIs would be preferrable to something that ran online period.