Preheat the oven to 350. Grease and flour two loaf pans.
Combine the milk and vinegar in a measuring cup. Allow the milk to curdle.
Combine the butter, sugar, eggs, lemon and orange zest, and milk.
Add the flour, baking soda and baking powder. Pour the batter into the prepared loaf pan and bake for one hour, or until a knife inserted in the center comes out clean.
While the cake is baking, prepare the syrup. Combine the sugar, rum, orange juice, and lemon juice.
Allow the cake to cool, then turn it out of the pan. Lay it on a piece of tin foil and pierce the top with a fork. Pour the syrup over the cake and wrap in the sheet of foil. Allow to rest overnight for the syrup to soak in.