Does a person have to decide on a date for zakat just like khums (i.e. December 31st, for example)?
If the year starts on January 1st for khums for example, then it will be due on December 31st, but zakat will be calculated/determined on December 1st. Determination of khums (i.e., spending, surplus) includes the last day of the year but for zakat it is applicable immediately when you start the 12th month. For further details, please consult the Manual of Islamic Laws.