Put the dates, vinegar, sugar, and 125 ml (4 fl oz/½ cup) of water in a small saucepan, bring to the boil then reduce the heat and simmer, stirring occasionally, for 6–8 minutes until the dates are very soft. Allow to cool slightly, then transfer the mixture to an upright blender and purée until smooth. Press the jam through a fine sieve into a small bowl, discarding any solids.