Creates an exchange rate by splitting the series at from (defaulting to today). The period covering that date is truncated; the new rate becomes the remainder through unbounded end (or the next period).

Currency exchange rates define the FX rate from a currency to the default currency over time. Rates for a currency form a contiguous series with no gaps: creating a rate splits the covering period, updating from moves the boundary with the predecessor, and deleting a rate expands a neighbor to fill the gap. The sole rate for a currency cannot be deleted.