diff --git a/mtl/evaluator.py b/mtl/evaluator.py
index 3f4b89f..5e57c85 100644
--- a/mtl/evaluator.py
+++ b/mtl/evaluator.py
@@ -125,7 +125,12 @@ def _min(val):
         return min(val[phi.arg])
 
     def _eval(x):
-        return f(x).rolling(a, b).map(_min, tag=phi)
+        tmp = f(x)
+        assert b >= a
+        if b > a:
+            return tmp.rolling(a, b).map(_min, tag=phi)
+
+        return tmp.retag({phi.arg: phi})
 
     return _eval
 
diff --git a/mtl/sugar.py b/mtl/sugar.py
index 54db20d..05a0b0e 100644
--- a/mtl/sugar.py
+++ b/mtl/sugar.py
@@ -25,5 +25,11 @@ def until(phi, psi):
     return ast.WeakUntil(phi, psi) & env(psi)
 
 
-def timed_until(phi, psi, lo, hi):
-    return env(psi, lo=lo, hi=hi) & alw(until(phi, psi), lo=0, hi=lo)
+def timed_until(left, right, lo, hi):
+    assert 0 <= lo < hi
+
+    expr = env(right, lo=lo, hi=hi)
+    expr &= alw(left, lo=0, hi=lo)
+    expr &= alw(until(left, right), lo=lo, hi=lo)
+
+    return expr