You have a complicated conditional (if-then-else) statement.
Extract methods from the condition, then part, and else parts.
if (date.before (SUMMER_START) || date.after(SUMMER_END)) charge = quantity * _winterRate + _winterServiceCharge; else charge = quantity * _summerRate;
if (notSummer(date)) charge = winterCharge(quantity); else charge = summerCharge (quantity);
For more inforamtion see page 238 of Refactoring