diff --git a/plugins/cost-insights/src/utils/change.test.ts b/plugins/cost-insights/src/utils/change.test.ts
index 94adc25f64..6a0a8e793b 100644
--- a/plugins/cost-insights/src/utils/change.test.ts
+++ b/plugins/cost-insights/src/utils/change.test.ts
@@ -76,7 +76,7 @@ describe('getPreviousPeriodTotalCost', () => {
       change: changeOf(MockAggregatedDailyCosts),
       trendline: trendlineOf(MockAggregatedDailyCosts),
     };
-    const exclusiveEndDate = '2020-10-01';
+    const exclusiveEndDate = '2020-09-30';
     expect(
       getPreviousPeriodTotalCost(
         mockGroupDailyCost,
diff --git a/plugins/cost-insights/src/utils/change.ts b/plugins/cost-insights/src/utils/change.ts
index 0fca672425..1718462948 100644
--- a/plugins/cost-insights/src/utils/change.ts
+++ b/plugins/cost-insights/src/utils/change.ts
@@ -22,6 +22,7 @@ import {
   GrowthType,
   MetricData,
   Duration,
+  DEFAULT_DATE_FORMAT,
 } from '../types';
 import dayjs, { OpUnitType } from 'dayjs';
 import duration from 'dayjs/plugin/duration';
@@ -68,13 +69,17 @@ export function getComparedChange(
 export function getPreviousPeriodTotalCost(
   dailyCost: Cost,
   duration: Duration,
-  endDate: string,
+  inclusiveEndDate: string,
 ): number {
   const dayjsDuration = dayjs.duration(duration);
-  const startDate = inclusiveStartDateOf(duration, endDate);
+  const startDate = inclusiveStartDateOf(
+    duration,
+    dayjs(inclusiveEndDate).add(1, 'day').format(DEFAULT_DATE_FORMAT),
+  );
+  // dayjs doesn't allow adding an ISO 8601 period to dates.
   const [amount, type]: [number, OpUnitType] = dayjsDuration.days()
-    ? [dayjsDuration.days(), 'days' as OpUnitType]
-    : [dayjsDuration.months(), 'months' as OpUnitType];
+    ? [dayjsDuration.days(), 'day']
+    : [dayjsDuration.months(), 'month'];
   const nextPeriodStart = dayjs(startDate).add(amount, type);
 
   // Add up costs that incurred before the start of the next period.
diff --git a/plugins/cost-insights/src/utils/duration.ts b/plugins/cost-insights/src/utils/duration.ts
index 6a880b382c..ae8c255b91 100644
--- a/plugins/cost-insights/src/utils/duration.ts
+++ b/plugins/cost-insights/src/utils/duration.ts
@@ -22,27 +22,27 @@ import { assertNever } from './assert';
  * Derive the start date of a given period, assuming two repeating intervals.
  *
  * @param duration see comment on Duration enum
- * @param endDate from CostInsightsApi.getLastCompleteBillingDate
+ * @param endDate from CostInsightsApi.getLastCompleteBillingDate + 1 day
  */
 export function inclusiveStartDateOf(
   duration: Duration,
-  endDate: string,
+  exclusiveEndDate: string,
 ): string {
   switch (duration) {
     case Duration.P30D:
     case Duration.P90D:
-      return moment(endDate)
+      return moment(exclusiveEndDate)
         .utc()
         .subtract(moment.duration(duration).add(moment.duration(duration)))
         .format(DEFAULT_DATE_FORMAT);
     case Duration.P1M:
-      return moment(endDate)
+      return moment(exclusiveEndDate)
         .utc()
         .startOf('month')
         .subtract(moment.duration(duration).add(moment.duration(duration)))
         .format(DEFAULT_DATE_FORMAT);
     case Duration.P3M:
-      return moment(endDate)
+      return moment(exclusiveEndDate)
         .utc()
         .startOf('quarter')
         .subtract(moment.duration(duration).add(moment.duration(duration)))
@@ -54,16 +54,22 @@ export function inclusiveStartDateOf(
 
 export function exclusiveEndDateOf(
   duration: Duration,
-  endDate: string,
+  inclusiveEndDate: string,
 ): string {
   switch (duration) {
     case Duration.P30D:
     case Duration.P90D:
-      return moment(endDate).utc().add(1, 'day').format(DEFAULT_DATE_FORMAT);
+      return moment(inclusiveEndDate)
+        .utc()
+        .add(1, 'day')
+        .format(DEFAULT_DATE_FORMAT);
     case Duration.P1M:
-      return moment(endDate).utc().startOf('month').format(DEFAULT_DATE_FORMAT);
+      return moment(inclusiveEndDate)
+        .utc()
+        .startOf('month')
+        .format(DEFAULT_DATE_FORMAT);
     case Duration.P3M:
-      return moment(endDate)
+      return moment(inclusiveEndDate)
         .utc()
         .startOf('quarter')
         .format(DEFAULT_DATE_FORMAT);
@@ -74,15 +80,15 @@ export function exclusiveEndDateOf(
 
 export function inclusiveEndDateOf(
   duration: Duration,
-  endDate: string,
+  inclusiveEndDate: string,
 ): string {
-  return moment(exclusiveEndDateOf(duration, endDate))
+  return moment(exclusiveEndDateOf(duration, inclusiveEndDate))
     .utc()
     .subtract(1, 'day')
     .format(DEFAULT_DATE_FORMAT);
 }
 
 // https://en.wikipedia.org/wiki/ISO_8601#Repeating_intervals
-export function intervalsOf(duration: Duration, endDate: string) {
-  return `R2/${duration}/${exclusiveEndDateOf(duration, endDate)}`;
+export function intervalsOf(duration: Duration, inclusiveEndDate: string) {
+  return `R2/${duration}/${exclusiveEndDateOf(duration, inclusiveEndDate)}`;
 }
diff --git a/plugins/cost-insights/src/utils/formatters.ts b/plugins/cost-insights/src/utils/formatters.ts
index bf8e26ef5f..e81fc1f7c9 100644
--- a/plugins/cost-insights/src/utils/formatters.ts
+++ b/plugins/cost-insights/src/utils/formatters.ts
@@ -15,7 +15,7 @@
  */
 
 import moment from 'moment';
-import { Duration } from '../types';
+import { Duration, DEFAULT_DATE_FORMAT } from '../types';
 import { inclusiveEndDateOf, inclusiveStartDateOf } from '../utils/duration';
 import { pluralOf } from '../utils/grammar';
 
@@ -84,21 +84,27 @@ export function formatPercent(n: number): string {
   return `${(n * 100).toFixed(0)}%`;
 }
 
-export function formatLastTwoLookaheadQuarters(endDate: string) {
-  const start = moment(inclusiveStartDateOf(Duration.P3M, endDate)).format(
-    '[Q]Q YYYY',
-  );
-  const end = moment(inclusiveEndDateOf(Duration.P3M, endDate)).format(
+export function formatLastTwoLookaheadQuarters(inclusiveEndDate: string) {
+  const exclusiveEndDate = moment(inclusiveEndDate)
+    .add(1, 'day')
+    .format(DEFAULT_DATE_FORMAT);
+  const start = moment(
+    inclusiveStartDateOf(Duration.P3M, exclusiveEndDate),
+  ).format('[Q]Q YYYY');
+  const end = moment(inclusiveEndDateOf(Duration.P3M, inclusiveEndDate)).format(
     '[Q]Q YYYY',
   );
   return `${start} vs ${end}`;
 }
 
-export function formatLastTwoMonths(endDate: string) {
-  const start = moment(inclusiveStartDateOf(Duration.P1M, endDate))
+export function formatLastTwoMonths(inclusiveEndDate: string) {
+  const exclusiveEndDate = moment(inclusiveEndDate)
+    .add(1, 'day')
+    .format(DEFAULT_DATE_FORMAT);
+  const start = moment(inclusiveStartDateOf(Duration.P1M, exclusiveEndDate))
     .utc()
     .format('MMMM');
-  const end = moment(inclusiveEndDateOf(Duration.P1M, endDate))
+  const end = moment(inclusiveEndDateOf(Duration.P1M, inclusiveEndDate))
     .utc()
     .format('MMMM');
   return `${start} vs ${end}`;