Saute onions, celery, garlic and ginger in oil in a heavy saucepan over medium heat for 3-4 minutes, or until translucent.
Remove from heat and stir in flour.
Cook for 1-2 minutes, then whisk in chicken broth.
Stir in carrots, cumin, curry powder, cayenne and black pepper.
Bring to a boil, reduce heat, cover and simmer for 10 mintes, stirring occasionally.
Stir in dates and simmer another 5-10 minutes or until carrots are soft.
Remove from heat and puree in blender or food processor until smooth.
Pour back into pan. Stir in lemon juice and heat until hot.
Spoon into bowls and garnish with a spoonful of yogurt.