diff --git a/extension/src/heartbeat_agg.rs b/extension/src/heartbeat_agg.rs
index 3656ae54..8d46fcd5 100644
--- a/extension/src/heartbeat_agg.rs
+++ b/extension/src/heartbeat_agg.rs
@@ -11,6 +11,8 @@ use crate::{
     ron_inout_funcs,
 };
 
+use std::cmp::{max, min};
+
 use toolkit_experimental::HeartbeatAggData;
 
 const BUFFER_SIZE: usize = 1000; // How many values to absorb before consolidating
@@ -80,6 +82,22 @@ impl HeartbeatTransState {
         }
     }
 
+    // In general we shouldn't need to change these creation time parameters, but if
+    // we're combining with another interval this may be necessary.
+    fn extend_covered_interval(&mut self, new_start: i64, new_end: i64) {
+        assert!(new_start <= self.start && new_end >= self.end); // this is guaranteed by the combine function
+        self.start = new_start;
+
+        // extend last range if able
+        if self.end < new_end && self.last + self.interval_len > self.end {
+            assert!(!self.liveness.is_empty()); // above condition should be impossible without liveness data
+
+            let mut last_interval = self.liveness.last_mut().unwrap();
+            last_interval.1 = min(self.last + self.interval_len, new_end);
+        }
+        self.end = new_end;
+    }
+
     fn combine_intervals(&mut self, new_intervals: Vec<(i64, i64)>) {
         // Optimized path for ordered inputs
         if self.liveness.last().unwrap().0 < new_intervals.first().unwrap().0 {
@@ -141,7 +159,14 @@ impl HeartbeatTransState {
         assert!(self.interval_len == other.interval_len); // Nicer error would be nice here
         self.process_batch();
         other.process_batch();
+
+        let min_start = min(self.start, other.start);
+        let max_end = max(self.end, other.end);
+        self.extend_covered_interval(min_start, max_end);
+        other.extend_covered_interval(min_start, max_end);
+
         self.combine_intervals(other.liveness);
+        self.last = max(self.last, other.last);
     }
 }
 
@@ -670,6 +695,52 @@ mod tests {
 
     #[pg_test]
     pub fn test_heartbeat_rollup() {
+        Spi::execute(|client| {
+            client.select("SET TIMEZONE to UTC", None, None);
+
+            client.select(
+                "CREATE TABLE heartbeats(time timestamptz, batch timestamptz)",
+                None,
+                None,
+            );
+
+            client.select(
+                "INSERT INTO heartbeats VALUES
+                    ('01-01-2020 3:02:20 UTC'::timestamptz, '01-01-2020 3:00:00 UTC'::timestamptz),
+                    ('01-01-2020 3:03:10 UTC'::timestamptz, '01-01-2020 3:00:00 UTC'::timestamptz),
+                    ('01-01-2020 3:04:07 UTC'::timestamptz, '01-01-2020 3:00:00 UTC'::timestamptz),
+                    ('01-01-2020 7:19:20 UTC'::timestamptz, '01-01-2020 7:00:00 UTC'::timestamptz),
+                    ('01-01-2020 7:39:20 UTC'::timestamptz, '01-01-2020 7:00:00 UTC'::timestamptz),
+                    ('01-01-2020 7:59:20 UTC'::timestamptz, '01-01-2020 7:00:00 UTC'::timestamptz),
+                    ('01-01-2020 8:00:10 UTC'::timestamptz, '01-01-2020 8:00:00 UTC'::timestamptz),
+                    ('01-01-2020 8:59:10 UTC'::timestamptz, '01-01-2020 8:00:00 UTC'::timestamptz),
+                    ('01-01-2020 23:34:20 UTC'::timestamptz, '01-01-2020 23:00:00 UTC'::timestamptz),
+                    ('01-01-2020 23:37:20 UTC'::timestamptz, '01-01-2020 23:00:00 UTC'::timestamptz),
+                    ('01-01-2020 23:38:05 UTC'::timestamptz, '01-01-2020 23:00:00 UTC'::timestamptz),
+                    ('01-01-2020 23:39:00 UTC'::timestamptz, '01-01-2020 23:00:00 UTC'::timestamptz)",
+                None,
+                None,
+            );
+
+            let result = client
+                .select(
+                    "WITH aggs AS (
+                    SELECT toolkit_experimental.heartbeat_agg(time, batch, '1h', '1m')
+                    FROM heartbeats 
+                    GROUP BY batch
+                ) SELECT toolkit_experimental.rollup(heartbeat_agg)::TEXT FROM aggs",
+                    None,
+                    None,
+                )
+                .first()
+                .get_one::<String>()
+                .unwrap();
+            assert_eq!("(version:1,start_time:631162800000000,end_time:631238400000000,last_seen:631237140000000,interval_len:60000000,num_intervals:7,interval_starts:[631162940000000,631178360000000,631179560000000,631180760000000,631184350000000,631236860000000,631237040000000],interval_ends:[631163107000000,631178420000000,631179620000000,631180870000000,631184410000000,631236920000000,631237200000000])", result);
+        })
+    }
+
+    #[pg_test]
+    pub fn test_heartbeat_combining_rollup() {
         Spi::execute(|client| {
             client.select("SET TIMEZONE to UTC", None, None);