fix: Data usage graphs don't work in UTC-n time zones (#9530)

Fixes a number of issues that would surface in UTC-n (where n > 1)
timezones. I've not found a way to check this with tests (and it looks
like [we weren't able to last time
either](https://github.com/Unleash/unleash/pull/9110/files#r1919746328)),
so all the testing's been done manually by adjusting my system time and
zone. (From what I understand, you can't generate a Date with a specific
TZ offset in JS: it's only utc or local time)

Resolved:
- [x] Selecting "Jan" in the dropdown results in the selection being
"December" (off by one in the selector)

- [x] Selecting a month view only gives you one data point (and it's
probably empty). Wrong date parsing on the way out resulted in sending
`{ from: "2025-02-28", to: "2025-02-28"}` instead of `{ from:
"2025-03-01", to: "2025-03-31"}`

- [x] The dates we create when making "daysRec" need to be adjusted.
They showed the wrong month, so the dates were off.

- [x] Make sure the labels are correct when hovering over. Again: we
used the wrong month for generating these.

- [x] The available months are wrong. Incorrect month parsing again.

- [x] The request summary month is wrong. You guessed it: incorrect
month parsing

frontend/src/component/admin/network/NetworkTrafficUsage/PeriodSelector.tsx

@@ -5,6 +5,7 @@ import { useRef, useState, type FC } from 'react';
 import { format } from 'date-fns';
 import type { ChartDataSelection } from './chart-data-selection';
 import { selectablePeriods } from './selectable-periods';
+import { parseMonthString } from './dates';
 
 const dropdownWidth = '15rem';
 const dropdownInlinePadding = (theme: Theme) => theme.spacing(3);
@@ -148,10 +149,13 @@ export const PeriodSelector: FC<Props> = ({ selectedPeriod, setPeriod }) => {
         selectedPeriod.grouping === 'daily'
             ? selectedPeriod.month === format(new Date(), 'yyyy-MM')
                 ? 'Current month'
-                : new Date(selectedPeriod.month).toLocaleDateString('en-US', {
+                : parseMonthString(selectedPeriod.month).toLocaleDateString(
-                      month: 'long',
+                      'en-US',
-                      year: 'numeric',
+                      {
-                  })
+                          month: 'long',
+                          year: 'numeric',
+                      },
+                  )
             : `Last ${selectedPeriod.monthsBack} months`;
 
     return (
@@ -184,7 +188,7 @@ export const PeriodSelector: FC<Props> = ({ selectedPeriod, setPeriod }) => {
                             <p>Last 12 months</p>
                         </MonthSelectorHeaderGroup>
                         <MonthGrid>
-                            {selectablePeriods.map((period, index) => (
+                            {selectablePeriods.map((period) => (
                                 <li key={period.label}>
                                     <GridButton
                                         selected={

frontend/src/component/admin/network/NetworkTrafficUsage/RequestSummary.tsx

@@ -4,6 +4,7 @@ import { subMonths } from 'date-fns';
 import { useLocationSettings } from 'hooks/useLocationSettings';
 import type { FC } from 'react';
 import type { ChartDataSelection } from './chart-data-selection';
+import { parseMonthString } from './dates';
 
 type Props = {
     period: ChartDataSelection;
@@ -62,7 +63,7 @@ const incomingRequestsText = (period: ChartDataSelection): string => {
         return `Average requests from ${formatMonth(fromMonth)} to ${formatMonth(toMonth)}`;
     }
 
-    return `Incoming requests in ${formatMonth(new Date(period.month))}`;
+    return `Incoming requests in ${formatMonth(parseMonthString(period.month))}`;
 };
 
 export const RequestSummary: FC<Props> = ({

frontend/src/component/admin/network/NetworkTrafficUsage/chart-data-selection.ts

@@ -1,4 +1,5 @@
 import { endOfMonth, format, startOfMonth, subMonths } from 'date-fns';
+import { parseMonthString } from './dates';
 
 export type ChartDataSelection =
     | {
@@ -16,7 +17,7 @@ export const toDateRange = (
 ): { from: string; to: string } => {
     const fmt = (date: Date) => format(date, 'yyyy-MM-dd');
     if (selection.grouping === 'daily') {
-        const month = new Date(selection.month);
+        const month = parseMonthString(selection.month);
         const from = fmt(month);
         const to = fmt(endOfMonth(month));
         return { from, to };

frontend/src/component/admin/network/NetworkTrafficUsage/chart-functions.ts

@@ -10,7 +10,7 @@ import {
     differenceInCalendarDays,
     differenceInCalendarMonths,
 } from 'date-fns';
-import { formatDay, formatMonth } from './dates';
+import { formatDay, formatMonth, parseDateString } from './dates';
 import type { ChartDataSelection } from './chart-data-selection';
 export type ChartDatasetType = ChartDataset<'bar'>;
 
@@ -80,8 +80,8 @@ const getLabelsAndRecords = (
     >,
 ) => {
     if (traffic.grouping === 'monthly') {
-        const from = new Date(traffic.dateRange.from);
+        const from = parseDateString(traffic.dateRange.from);
-        const to = new Date(traffic.dateRange.to);
+        const to = parseDateString(traffic.dateRange.to);
         const numMonths = Math.abs(differenceInCalendarMonths(to, from)) + 1;
         const monthsRec: { [month: string]: number } = {};
         for (let i = 0; i < numMonths; i++) {
@@ -95,8 +95,8 @@ const getLabelsAndRecords = (
         );
         return { newRecord: () => ({ ...monthsRec }), labels };
     } else {
-        const from = new Date(traffic.dateRange.from);
+        const from = parseDateString(traffic.dateRange.from);
-        const to = new Date(traffic.dateRange.to);
+        const to = parseDateString(traffic.dateRange.to);
         const numDays = Math.abs(differenceInCalendarDays(to, from)) + 1;
         const daysRec: { [day: string]: number } = {};
         for (let i = 0; i < numDays; i++) {

frontend/src/component/admin/network/NetworkTrafficUsage/dates.ts

@@ -1,4 +1,4 @@
-import { format, getDaysInMonth } from 'date-fns';
+import { format, getDaysInMonth, parse } from 'date-fns';
 
 export const currentDate = new Date();
 
@@ -11,3 +11,13 @@ export const daysInCurrentMonth = getDaysInMonth(currentDate);
 
 export const formatMonth = (date: Date) => format(date, 'yyyy-MM');
 export const formatDay = (date: Date) => format(date, 'yyyy-MM-dd');
+
+export const parseMonthString = (month: string): Date => {
+    // parses a month into a Date starting on the first day of the month, regardless of the current time zone (e.g. works in Norway and Brazil)
+    return parse(month, 'yyyy-MM', new Date());
+};
+
+export const parseDateString = (month: string): Date => {
+    // parses a date string into a Date, regardless of the current time zone (e.g. works in Norway and Brazil)
+    return parse(month, 'yyyy-MM-dd', new Date());
+};

frontend/src/component/admin/network/NetworkTrafficUsage/hooks/useChartDataSelection.ts

@@ -4,6 +4,7 @@ import { periodsRecord, selectablePeriods } from '../selectable-periods';
 import { createBarChartOptions } from '../bar-chart-options';
 import useTheme from '@mui/material/styles/useTheme';
 import { useLocationSettings } from 'hooks/useLocationSettings';
+import { parseMonthString } from '../dates';
 
 export const useChartDataSelection = (includedTraffic?: number) => {
     const theme = useTheme();
@@ -35,11 +36,11 @@ export const useChartDataSelection = (includedTraffic?: number) => {
                         },
                     );
                 } else {
-                    const timestamp = Date.parse(tooltipItems[0].label);
+                    const month = parseMonthString(tooltipItems[0].label);
-                    if (Number.isNaN(timestamp)) {
+                    if (Number.isNaN(month.getTime())) {
                         return 'Current month to date';
                     }
-                    return new Date(timestamp).toLocaleDateString(
+                    return month.toLocaleDateString(
                         locationSettings?.locale ?? 'en-US',
                         {
                             month: 'long',

frontend/src/component/admin/network/NetworkTrafficUsage/hooks/useStats.ts

@@ -3,8 +3,8 @@ import { useTrafficSearch } from 'hooks/api/getters/useInstanceTrafficMetrics/us
 import { currentDate } from '../dates';
 import { useMemo } from 'react';
 import {
-    toTrafficUsageChartData as newToChartData,
     toConnectionChartData,
+    toTrafficUsageChartData,
 } from '../chart-functions';
 import {
     calculateEstimatedMonthlyCost,
@@ -36,7 +36,7 @@ export const useTrafficStats = (
         }
         const traffic = result.data;
 
-        const chartData = newToChartData(traffic, filter);
+        const chartData = toTrafficUsageChartData(traffic, filter);
         const usageTotal = calculateTotalUsage(traffic);
         const overageCost = calculateOverageCost(
             usageTotal,

frontend/src/component/admin/network/NetworkTrafficUsage/selectable-periods.ts

@@ -1,4 +1,4 @@
-import { getDaysInMonth } from 'date-fns';
+import { getDaysInMonth, startOfMonth, subMonths } from 'date-fns';
 import { currentDate, formatMonth } from './dates';
 import { TRAFFIC_MEASUREMENT_START_DATE } from 'utils/traffic-calculations';
 
@@ -38,21 +38,18 @@ export const toSelectablePeriod = (
 
 export const generateSelectablePeriodsFromDate = (now: Date) => {
     const selectablePeriods = [toSelectablePeriod(now, 'Current month')];
+    const startOfCurrentMonth = startOfMonth(now);
     for (
         let subtractMonthCount = 1;
         subtractMonthCount < 12;
         subtractMonthCount++
     ) {
-        // this complicated calc avoids DST issues
+        const targetMonth = subMonths(startOfCurrentMonth, subtractMonthCount);
-        const utcYear = now.getUTCFullYear();
-        const utcMonth = now.getUTCMonth();
-        const targetMonth = utcMonth - subtractMonthCount;
-        const targetDate = new Date(Date.UTC(utcYear, targetMonth, 1, 0, 0, 0));
         selectablePeriods.push(
             toSelectablePeriod(
-                targetDate,
+                targetMonth,
                 undefined,
-                targetDate >= TRAFFIC_MEASUREMENT_START_DATE,
+                targetMonth >= TRAFFIC_MEASUREMENT_START_DATE,
             ),
         );
     }

frontend/src/utils/traffic-calculations.ts

@@ -4,10 +4,11 @@ import type {
 } from 'openapi';
 import { getDaysInMonth } from 'date-fns';
 import { format } from 'date-fns';
+import { parseDateString } from 'component/admin/network/NetworkTrafficUsage/dates';
 export const DEFAULT_TRAFFIC_DATA_UNIT_COST = 5;
 export const DEFAULT_TRAFFIC_DATA_UNIT_SIZE = 1_000_000;
 
-export const TRAFFIC_MEASUREMENT_START_DATE = new Date('2024-05-01');
+export const TRAFFIC_MEASUREMENT_START_DATE = parseDateString('2024-05-01');
 
 export const METERED_TRAFFIC_ENDPOINTS = [
     '/api/admin',