Cut the rhubarb into chunks roughly ¾ inch wide, then put into a blender with ½ cup water. Whirl into a raspberry-colored puree. It won’t be totally smooth but do your best to minimize stringiness.
Scrape into a small saucepan and bring to a vigorous simmer over medium heat. Lower the heat to cook (expect some sputtering) for 5 minutes, until thickened with no liquid separating. Remove from the heat and let cool for about 10 minutes. You should net about ¾ cup.
Meanwhile, rinse the blender..Return the rhubarb to the cleaned blender, then add ⅓ cup water plus the dates, sugar, salt, and cayenne. Blend to a smooth, thick texture. As needed, pause the machine to scrape down the sides. Ideally, the chutney is thick enough to mound on a spoon. (If you want a thinner sauce, add water by the tablespoon but you may need to adjust the other ingredients too.)
Transfer to a bowl (or airtight container) and stir in the cumin. Taste and if it is too sweet, stir in the vinegar. Let sit for about 2 hours to allow flavors to fully develop. Serve, refrigerate for up to 2 weeks; I’m sure this is freezable for a month too!