Heat the oven to 350°F (175°C). Grease and line an 8-inch or 9-inch square baking pan with parchment paper, leaving enough overhang to lift out the cooled squares.

Combine the chopped dates, water, granulated sugar, and lemon juice in a saucepan. Simmer over medium-low heat for 5–8 minutes, stirring until the dates soften into a thick, spreadable paste, then stir in the vanilla.

In a large bowl, stir together the flour, rolled oats, brown sugar, baking soda, and salt. Add the melted butter and mix until the dry ingredients are evenly moistened and crumbly.

Press about half of the crumble mixture firmly into the prepared pan. Spread the warm date filling evenly over the base.

Sprinkle the remaining crumble mixture over the date layer. Press it very lightly so the topping holds together without compacting it into a dense crust.

Bake for 35–40 minutes, until the top is golden brown. Cool completely before cutting because the date filling firms as it cools.