diff --git a/runtime/pkg/rilltime/rilltime.go b/runtime/pkg/rilltime/rilltime.go
index 37999eb8a36e..8ce5cbb06601 100644
--- a/runtime/pkg/rilltime/rilltime.go
+++ b/runtime/pkg/rilltime/rilltime.go
@@ -76,6 +76,16 @@ var (
 		"PQ": "-1Q/Q to ref/Q",
 		"PY": "-1Y/Y to ref/Y",
 	}
+	// Mapping for our old rill-<DAX> comparison offsets to ISO durations.
+	// Older reports/alerts may send these as the offset of a legacy comparison time range.
+	// "PP" (previous period) is handled separately since it depends on the main interval.
+	daxOffsetNotations = map[string]string{
+		"PD": "P1D",
+		"PW": "P1W",
+		"PM": "P1M",
+		"PQ": "P3M",
+		"PY": "P1Y",
+	}
 	grainMap = map[string]timeutil.TimeGrain{
 		"s": timeutil.TimeGrainSecond,
 		"S": timeutil.TimeGrainSecond,
@@ -352,7 +362,19 @@ func ParseLegacy(duration, offset string, roundToGrain timeutil.TimeGrain, parse
 
 	if offset != "" {
 		if strings.HasPrefix(offset, "rill-") {
-			return nil, fmt.Errorf("offset cannot have DAX notation")
+			dax := strings.TrimPrefix(offset, "rill-")
+			if dax == "PP" {
+				rt.Offset = &Offset{
+					PreviousPeriod: &PreviousPeriod{Prefix: "-", Num: 1},
+				}
+				return rt, nil
+			}
+
+			iso, ok := daxOffsetNotations[dax]
+			if !ok {
+				return nil, fmt.Errorf("invalid DAX offset %q", offset)
+			}
+			offset = iso
 		}
 
 		offsetGrainPart, err := parseISODuration(offset)
diff --git a/runtime/pkg/rilltime/rilltime_test.go b/runtime/pkg/rilltime/rilltime_test.go
index bae3ff6a59e0..5860cc8fbe1d 100644
--- a/runtime/pkg/rilltime/rilltime_test.go
+++ b/runtime/pkg/rilltime/rilltime_test.go
@@ -495,6 +495,14 @@ func TestParseISO(t *testing.T) {
 		{"With duration and offset no round to grain", "P7D", "P2D", timeutil.TimeGrainUnspecified, "2025-05-04T06:32:36Z", "2025-05-11T06:32:36Z", timeutil.TimeGrainUnspecified},
 		{"With duration, offset and round to grain", "P7D", "P2D", timeutil.TimeGrainDay, "2025-05-04T00:00:00Z", "2025-05-11T00:00:00Z", timeutil.TimeGrainUnspecified},
 		{"With DAX duration, offset and round to grain", "rill-PW", "P2D", timeutil.TimeGrainDay, "2025-05-03T00:00:00Z", "2025-05-10T00:00:00Z", timeutil.TimeGrainUnspecified},
+		// Legacy DAX comparison offsets sent by older clients
+		{"With duration and DAX previous period offset", "P7D", "rill-PP", timeutil.TimeGrainUnspecified, "2025-04-29T06:32:36Z", "2025-05-06T06:32:36Z", timeutil.TimeGrainUnspecified},
+		{"With duration and DAX previous day offset", "P7D", "rill-PD", timeutil.TimeGrainUnspecified, "2025-05-05T06:32:36Z", "2025-05-12T06:32:36Z", timeutil.TimeGrainUnspecified},
+		{"With duration and DAX previous week offset", "P7D", "rill-PW", timeutil.TimeGrainUnspecified, "2025-04-29T06:32:36Z", "2025-05-06T06:32:36Z", timeutil.TimeGrainUnspecified},
+		{"With duration and DAX previous month offset", "P7D", "rill-PM", timeutil.TimeGrainUnspecified, "2025-04-06T06:32:36Z", "2025-04-13T06:32:36Z", timeutil.TimeGrainUnspecified},
+		{"With duration and DAX previous quarter offset", "P7D", "rill-PQ", timeutil.TimeGrainUnspecified, "2025-02-06T06:32:36Z", "2025-02-13T06:32:36Z", timeutil.TimeGrainUnspecified},
+		{"With duration and DAX previous year offset", "P7D", "rill-PY", timeutil.TimeGrainUnspecified, "2024-05-06T06:32:36Z", "2024-05-13T06:32:36Z", timeutil.TimeGrainUnspecified},
+		{"With duration, DAX previous week offset and round to grain", "P7D", "rill-PW", timeutil.TimeGrainDay, "2025-04-29T00:00:00Z", "2025-05-06T00:00:00Z", timeutil.TimeGrainUnspecified},
 	}
 
 	nowTm := parseTestTime(t, now)
@@ -522,6 +530,11 @@ func TestParseISO(t *testing.T) {
 	}
 }
 
+func TestParseISO_InvalidOffset(t *testing.T) {
+	_, err := ParseLegacy("P7D", "rill-PX", timeutil.TimeGrainUnspecified, ParseOptions{})
+	require.ErrorContains(t, err, `invalid DAX offset "rill-PX"`)
+}
+
 func TestEval_SyntaxErrors(t *testing.T) {
 	testCases := []struct {
 		timeRange string