diff --git a/src/openstack_billing_db/billing.py b/src/openstack_billing_db/billing.py
index 7c1b7c7..e3fed37 100644
--- a/src/openstack_billing_db/billing.py
+++ b/src/openstack_billing_db/billing.py
@@ -8,11 +8,18 @@
 import os
 
 from openstack_billing_db import model
+from openstack_billing_db import utils
 
 import boto3
 
 logger = logging.getLogger(__name__)
 
+# FIXME(knikolla): Temporarily hardcoding the days of outage here
+# until we have formalized how to store them in the nerc-rates repo.
+# Usage during these intervals is subtracted from the usage during
+# the month.
+OUTAGES_FOR_MONTH = {"2024-05": [("2024-05-22", "2024-05-29")]}
+
 
 @dataclass()
 class Rates(object):
@@ -81,7 +88,26 @@ def gpu_a2_su_cost(self) -> Decimal:
         return self.rates.gpu_a2 * self.gpu_a2_su_hours
 
 
-def collect_invoice_data_from_openstack(database, billing_start, billing_end, rates):
+def get_runtime_for_instance(
+    instance: model.Instance,
+    start: datetime,
+    end: datetime,
+    excluded_intervals: list[(str, str)],
+):
+    runtime = instance.get_runtime_during(start, end)
+    for interval in excluded_intervals:
+        excluded_runtime = instance.get_runtime_during(
+            start_time=utils.parse_time_from_string(interval[0]),
+            end_time=utils.parse_time_from_string(interval[1]),
+        )
+        runtime = runtime - excluded_runtime
+
+    return runtime
+
+
+def collect_invoice_data_from_openstack(
+    database, billing_start, billing_end, rates, invoice_month=None
+):
     invoices = []
     for project in database.projects:
         invoice = ProjectInvoice(
@@ -94,8 +120,14 @@ def collect_invoice_data_from_openstack(database, billing_start, billing_end, ra
             rates=rates,
         )
 
+        excluded_intervals = []
+        if invoice_month:
+            excluded_intervals = OUTAGES_FOR_MONTH.get(invoice_month, [])
+
         for i in project.instances:  # type: model.Instance
-            runtime = i.get_runtime_during(billing_start, billing_end)
+            runtime = get_runtime_for_instance(
+                i, billing_start, billing_end, excluded_intervals
+            )
             runtime_seconds = runtime.total_seconds_running
             if rates.include_stopped_runtime:
                 runtime_seconds += runtime.total_seconds_stopped
@@ -221,7 +253,9 @@ def generate_billing(
 ):
     database = model.Database(start, sql_dump_file)
 
-    invoices = collect_invoice_data_from_openstack(database, start, end, rates)
+    invoices = collect_invoice_data_from_openstack(
+        database, start, end, rates, invoice_month=invoice_month
+    )
     if coldfront_data_file:
         merge_coldfront_data(invoices, coldfront_data_file)
     write(invoices, output, invoice_month)
diff --git a/src/openstack_billing_db/main.py b/src/openstack_billing_db/main.py
index c5f2aef..37f6e55 100644
--- a/src/openstack_billing_db/main.py
+++ b/src/openstack_billing_db/main.py
@@ -4,7 +4,7 @@
 import argparse
 import logging
 
-from openstack_billing_db import billing, fetch
+from openstack_billing_db import billing, fetch, utils
 
 logging.basicConfig(level=logging.INFO)
 logger = logging.getLogger(__name__)
@@ -12,7 +12,7 @@
 
 def parse_time_argument(arg):
     if isinstance(arg, str):
-        return datetime.strptime(arg, "%Y-%m-%d")
+        return utils.parse_time_from_string(arg)
     return arg
 
 
diff --git a/src/openstack_billing_db/model.py b/src/openstack_billing_db/model.py
index 8b9cc21..b248391 100644
--- a/src/openstack_billing_db/model.py
+++ b/src/openstack_billing_db/model.py
@@ -69,6 +69,12 @@ class InstanceRuntime(object):
     total_seconds_running: int = 0
     total_seconds_stopped: int = 0
 
+    def __sub__(self, other):
+        return InstanceRuntime(
+            self.total_seconds_running - other.total_seconds_running,
+            self.total_seconds_stopped - other.total_seconds_stopped,
+        )
+
 
 @dataclass
 class Instance(object):
@@ -100,20 +106,15 @@ def get_runtime_during(self, start_time, end_time):
         in_error_state = False
         delete_action_found = False
 
-        # If the instance as a deleted_at time, clamp it to within
-        # the invoicing period.
-        if self.deleted_at:
-            self.deleted_at = self._clamp_time(self.deleted_at, start_time, end_time)
-
         for event in self.events:
-            event.time = self._clamp_time(event.time, start_time, end_time)
+            event_time = self._clamp_time(event.time, start_time, end_time)
 
             if event.message == "Error":
                 in_error_state = True
                 continue
 
             if event.name in ["create", "start"]:
-                last_start = event.time
+                last_start = event_time
                 in_error_state = False
 
                 # Count stopped time from last known stop.
@@ -129,7 +130,7 @@ def get_runtime_during(self, start_time, end_time):
                 delete_action_found = True
 
             if event.name in ["delete", "stop"]:
-                last_stop = event.time
+                last_stop = event_time
 
                 # Count running time from last known start.
                 if last_start:
@@ -146,7 +147,7 @@ def get_runtime_during(self, start_time, end_time):
 
         if self.deleted_at and not delete_action_found:
             self.no_delete_action = True
-            end_time = self.deleted_at
+            end_time = self._clamp_time(self.deleted_at, start_time, end_time)
 
         # Handle the time since the last event.
         if last_start:
diff --git a/src/openstack_billing_db/tests/unit/test_billing.py b/src/openstack_billing_db/tests/unit/test_billing.py
new file mode 100644
index 0000000..d30c703
--- /dev/null
+++ b/src/openstack_billing_db/tests/unit/test_billing.py
@@ -0,0 +1,29 @@
+import uuid
+from datetime import datetime, timedelta
+
+from openstack_billing_db import billing
+from openstack_billing_db.model import Instance, InstanceEvent
+from openstack_billing_db.tests.unit.utils import FLAVORS, MINUTE, HOUR, DAY, MONTH
+
+
+def test_instance_simple_runtime():
+    time = datetime(year=2000, month=1, day=1, hour=0, minute=0, second=0)
+    events = [
+        InstanceEvent(time=time, name="create", message=""),
+        InstanceEvent(time=time + timedelta(days=15), name="delete", message=""),
+    ]
+    i = Instance(
+        uuid=uuid.uuid4().hex, name=uuid.uuid4().hex, flavor=FLAVORS[1], events=events
+    )
+
+    r = billing.get_runtime_for_instance(
+        i,
+        datetime(year=2000, month=1, day=1, hour=0, minute=0, second=0),
+        datetime(year=2000, month=2, day=1, hour=0, minute=0, second=0),
+        excluded_intervals=[
+            ["2000-01-07", "2000-01-08"],
+            ["2000-01-01", "2000-01-02"],
+        ],
+    )
+    assert r.total_seconds_running == (15 * DAY) - (DAY * 2)
+    assert r.total_seconds_stopped == 0
diff --git a/src/openstack_billing_db/tests/unit/test_instance.py b/src/openstack_billing_db/tests/unit/test_instance.py
index c5753d0..a7fb0c4 100644
--- a/src/openstack_billing_db/tests/unit/test_instance.py
+++ b/src/openstack_billing_db/tests/unit/test_instance.py
@@ -1,14 +1,8 @@
 import uuid
 from datetime import datetime, timedelta
 
-from openstack_billing_db.model import Instance, InstanceEvent, Flavor
-
-FLAVORS = {1: Flavor(id=1, name="TestFlavor", vcpus=1, memory=4096, storage=10)}
-
-MINUTE = 60
-HOUR = 60 * MINUTE
-DAY = HOUR * 24
-MONTH = 31 * DAY
+from openstack_billing_db.model import Instance, InstanceEvent
+from openstack_billing_db.tests.unit.utils import FLAVORS, MINUTE, HOUR, DAY, MONTH
 
 
 def test_instance_simple_runtime():
@@ -115,7 +109,7 @@ def test_instance_no_delete_action():
 
     r = i.get_runtime_during(
         datetime(year=1999, month=11, day=1, hour=0, minute=0, second=0),
-        datetime(year=2000, month=12, day=1, hour=0, minute=0, second=0),
+        datetime(year=1999, month=12, day=1, hour=0, minute=0, second=0),
     )
     assert r.total_seconds_running == 0
     assert r.total_seconds_stopped == 0
diff --git a/src/openstack_billing_db/tests/unit/test_instance_runtime.py b/src/openstack_billing_db/tests/unit/test_instance_runtime.py
new file mode 100644
index 0000000..1283f34
--- /dev/null
+++ b/src/openstack_billing_db/tests/unit/test_instance_runtime.py
@@ -0,0 +1,11 @@
+from openstack_billing_db.model import InstanceRuntime
+
+
+def test_instance_runtime_subtract():
+    a = InstanceRuntime(total_seconds_running=1000, total_seconds_stopped=1000)
+    b = InstanceRuntime(total_seconds_running=100, total_seconds_stopped=200)
+    c = a - b
+    assert c.total_seconds_running == 900
+    assert c.total_seconds_running == a.total_seconds_running - b.total_seconds_running
+    assert c.total_seconds_stopped == 800
+    assert c.total_seconds_stopped == a.total_seconds_stopped - b.total_seconds_stopped
diff --git a/src/openstack_billing_db/tests/unit/utils.py b/src/openstack_billing_db/tests/unit/utils.py
new file mode 100644
index 0000000..9359027
--- /dev/null
+++ b/src/openstack_billing_db/tests/unit/utils.py
@@ -0,0 +1,8 @@
+from openstack_billing_db import model
+
+FLAVORS = {1: model.Flavor(id=1, name="TestFlavor", vcpus=1, memory=4096, storage=10)}
+
+MINUTE = 60
+HOUR = 60 * MINUTE
+DAY = HOUR * 24
+MONTH = 31 * DAY
diff --git a/src/openstack_billing_db/utils.py b/src/openstack_billing_db/utils.py
new file mode 100644
index 0000000..95447a3
--- /dev/null
+++ b/src/openstack_billing_db/utils.py
@@ -0,0 +1,5 @@
+from datetime import datetime
+
+
+def parse_time_from_string(time_str: str) -> datetime:
+    return datetime.strptime(time_str, "%Y-%m-%d")