Preheat the oven to 180 degrees C. Prepare a 450g bread tin by lining the bottom with grease proof paper and greasing the sides.
Cream butter and sugar together with an electric whisk until it's light and fluffy. Beat in the eggs one at a time (it's better if they are room temperature to prevent curdling of the mixture). Mix in the milk. Sift the flour and baking powder in and mix until well combined.
Put the dates in a saucepan with a bit of water; about 100ml. Heat on a medium to low heat until they are soft and pulpy; about 10 minutes. Remove from heat and mash with a fork.
Mash the bananas in bowl and add about 1/4 to the dates and mix it all together. Add the remaining 3/4 to the flour mixture and combine.
Put 1/3 of the flour mixture into the loaf tin and then layer on top 1/2 of the date mixture. Repeat until there is no mixture left and you finish with the flour mixture on top.
You can add some dates sliced lengthways to the top for aesthetic reason if you wish but they can burn and be a bit chewy.
Put in middle shelf of oven for an hour to an hour and a half. To test put a skewer into middle and it should come out without uncooked cake mixture but not completely clean depending on how moist you want your cake.