Stats: fix CSV export filenames for selected date ranges

Use the custom range query dates (and period) for download filenames so exports match the selected selection instead of a single period unit.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Nikschavan

client/my-sites/stats/stats-download-csv/get-stats-csv-filename.js

@@ -0,0 +1,28 @@
+import moment from 'moment';
+
+/**
+ * Builds the CSV export filename so it matches the data selection.
+ *
+ * When a custom date range query is present (`start_date` + `date`), those dates
+ * and the query period are used. Otherwise the legacy period object bounds are used.
+ *
+ * @param {Object} options
+ * @param {string} options.siteSlug Site slug used as the filename prefix.
+ * @param {string} options.path Stats module path segment (e.g. "posts").
+ * @param {Object} options.period Period object with `period`, `startOf`, and `endOf`.
+ * @param {Object} [options.query] Stats query; custom ranges include `start_date` and `date`.
+ * @returns {string} Filename ending in `.csv`.
+ */
+export function getStatsCsvFileName( { siteSlug, path, period, query } ) {
+	const hasCustomDateRange = Boolean( query?.start_date && query?.date );
+	const periodLabel = hasCustomDateRange && query.period ? query.period : period.period;
+
+	const startDate = hasCustomDateRange
+		? moment( query.start_date, 'YYYY-MM-DD' ).format( 'L' )
+		: period.startOf.format( 'L' );
+	const endDate = hasCustomDateRange
+		? moment( query.date, 'YYYY-MM-DD' ).format( 'L' )
+		: period.endOf.format( 'L' );
+
+	return [ siteSlug, path, periodLabel, startDate, endDate ].join( '-' ) + '.csv';
+}

client/my-sites/stats/stats-download-csv/index.jsx

@@ -16,6 +16,7 @@ import {
 	isRequestingSiteStatsForQuery,
 } from 'calypso/state/stats/lists/selectors';
 import { getSelectedSiteId } from 'calypso/state/ui/selectors';
+import { getStatsCsvFileName } from './get-stats-csv-filename';
 
 import './style.scss';
 
@@ -53,16 +54,9 @@ class StatsDownloadCsv extends Component {
 
 	downloadCsv = ( event ) => {
 		event.preventDefault();
-		const { siteSlug, path, period, data, headers } = this.props;
-
-		const fileName =
-			[
-				siteSlug,
-				path,
-				period.period,
-				period.startOf.format( 'L' ),
-				period.endOf.format( 'L' ),
-			].join( '-' ) + '.csv';
+		const { siteSlug, path, period, query, data, headers } = this.props;
+
+		const fileName = getStatsCsvFileName( { siteSlug, path, period, query } );
 
 		this.props.recordGoogleEvent( 'Stats', 'CSV Download ' + titlecase( path ) );
 

client/my-sites/stats/stats-download-csv/test/get-stats-csv-filename.js

@@ -0,0 +1,79 @@
+import moment from 'moment';
+import { getStatsCsvFileName } from '../get-stats-csv-filename';
+
+describe( 'getStatsCsvFileName', () => {
+	const siteSlug = 'mercantile.wordpress.org';
+	const path = 'posts';
+
+	beforeAll( () => {
+		moment.locale( 'en' );
+	} );
+
+	it( 'uses the selected custom date range for multi-month exports (STATS-420)', () => {
+		// Period object still reflects a single month unit around the start date,
+		// which previously produced filenames like posts-month-01/01/2026-01/31/2026.
+		const period = {
+			period: 'month',
+			startOf: moment( '2026-01-01' ),
+			endOf: moment( '2026-01-31' ),
+		};
+		const query = {
+			period: 'day',
+			start_date: '2026-01-01',
+			date: '2026-08-06',
+			summarize: 1,
+		};
+
+		expect( getStatsCsvFileName( { siteSlug, path, period, query } ) ).toBe(
+			'mercantile.wordpress.org-posts-day-01/01/2026-08/06/2026.csv'
+		);
+	} );
+
+	it( 'uses the selected custom date range for a single-month export (STATS-420)', () => {
+		// A day-period unit around the start date previously produced
+		// posts-day-01/01/2026-01/01/2026 for a Jan 1–31 selection.
+		const period = {
+			period: 'day',
+			startOf: moment( '2026-01-01' ),
+			endOf: moment( '2026-01-01' ),
+		};
+		const query = {
+			period: 'day',
+			start_date: '2026-01-01',
+			date: '2026-01-31',
+			summarize: 1,
+		};
+
+		expect( getStatsCsvFileName( { siteSlug, path, period, query } ) ).toBe(
+			'mercantile.wordpress.org-posts-day-01/01/2026-01/31/2026.csv'
+		);
+	} );
+
+	it( 'falls back to period bounds when no custom date range is present', () => {
+		const period = {
+			period: 'month',
+			startOf: moment( '2026-01-01' ),
+			endOf: moment( '2026-01-31' ),
+		};
+		const query = {
+			period: 'month',
+			date: '2026-01-31',
+		};
+
+		expect( getStatsCsvFileName( { siteSlug, path, period, query } ) ).toBe(
+			'mercantile.wordpress.org-posts-month-01/01/2026-01/31/2026.csv'
+		);
+	} );
+
+	it( 'falls back to period bounds when query is omitted', () => {
+		const period = {
+			period: 'week',
+			startOf: moment( '2026-01-05' ),
+			endOf: moment( '2026-01-11' ),
+		};
+
+		expect( getStatsCsvFileName( { siteSlug, path, period } ) ).toBe(
+			'mercantile.wordpress.org-posts-week-01/05/2026-01/11/2026.csv'
+		);
+	} );
+} );