use the DateTime object when comparing dates in ExhcnageRateCalculator

src/Finance/ExchangeRateCalculator.php

@@ -41,7 +41,7 @@ class ExchangeRateCalculator
 
         $currentExchangeRate = null;
         foreach ($this->exchangeRates[$currency->getId()] as $exchangeRate) {
-            if ($exchangeRate->getValidFrom() > $time) {
+            if ($exchangeRate->getValidFromDate() > $time) {
                 break;
             }
             $currentExchangeRate = $exchangeRate;