diff --git a/algorithm-catalog/data/events.json b/algorithm-catalog/data/events.json
index 259f30c8..1cf84a6d 100644
--- a/algorithm-catalog/data/events.json
+++ b/algorithm-catalog/data/events.json
@@ -199,7 +199,6 @@
     "name": "Hurricane Isaias October 2026",
     "stacName": "202610_Hurricane_Isaias",
     "start": "2026-10-09",
-    "end": "2026-10-09",
     "hazards": ["TropicalCyclone", "Flood"],
     "locations": ["Florida", "Alabama"],
     "places": ["US-FL", "US-AL"],
diff --git a/algorithm-catalog/scripts/e2e.mjs b/algorithm-catalog/scripts/e2e.mjs
index 54b4c609..c14ccb65 100644
--- a/algorithm-catalog/scripts/e2e.mjs
+++ b/algorithm-catalog/scripts/e2e.mjs
@@ -222,6 +222,17 @@ await page.goto(BASE, { waitUntil: "networkidle" });
 await page.locator("button", { hasText: /event/i }).first().click();
 await page.waitForTimeout(300);
 check("imported events are listed (8)", (await page.locator(".ecard").count()) === 8);
+check(
+  "an event with no end date reads as ongoing",
+  /9 Oct 2026 – ongoing/.test(await page.locator(".ecard", { hasText: "Hurricane Isaias" }).innerText()),
+);
+await page.locator('input[type="date"]').first().fill("2026-10-15");
+await page.waitForTimeout(300);
+check(
+  "an ongoing event still matches a later date filter",
+  (await page.locator(".ecard", { hasText: "Hurricane Isaias" }).count()) === 1,
+);
+await page.locator("button", { hasText: "Reset" }).click();
 await page.locator(".ecard", { hasText: "Hurricane Isaias October 2026" }).click();
 await page.locator('[role="tab"]', { hasText: "Page preview" }).click();
 const preview = await page.locator(".page-preview").innerText();
@@ -234,6 +245,7 @@ await page.locator("button", { hasText: "Edit page" }).click();
 await page.waitForTimeout(400);
 check("Edit page opens Submit prefilled", (await page.locator("#req-name").inputValue()) === "Hurricane Isaias October 2026");
 check("…with the event's places as chips", (await page.locator(".place-chip").count()) === 2);
+check("…and the end date blank (optional — ongoing)", (await page.locator("#req-end").inputValue()) === "");
 await page.locator("#req-subtitle").fill('As Hurricane Isaias made landfall, NASA "activated" to support FEMA.');
 await page.locator("button.req-haz", { hasText: /^Respond$/ }).click();
 await page.locator("#req-intro").fill("Hurricane Isaias brought storm surge to Florida and Alabama.\n\nNASA is sharing maps.");
@@ -257,7 +269,10 @@ const opened = await page.evaluate(() => window.__opened[0] || "");
 const doc = opened ? JSON.parse(new URL(opened).searchParams.get("value"))[0] : {};
 check(
   "Open PR carries the page + places in the request file",
-  doc.page?.intro?.startsWith("Hurricane Isaias") && doc.page?.needs?.[0] === "respond" && doc.event?.places?.length === 2,
+  doc.page?.intro?.startsWith("Hurricane Isaias") &&
+    doc.page?.needs?.[0] === "respond" &&
+    doc.event?.places?.length === 2 &&
+    !("end" in (doc.event ?? {})),
   opened ? new URL(opened).searchParams.get("filename") : "no window.open",
 );
 
diff --git a/algorithm-catalog/scripts/import_xlsx.py b/algorithm-catalog/scripts/import_xlsx.py
index cb95d869..f9d65d8e 100644
--- a/algorithm-catalog/scripts/import_xlsx.py
+++ b/algorithm-catalog/scripts/import_xlsx.py
@@ -288,16 +288,16 @@ def main(argv: list[str] | None = None) -> int:
             report.append(f"{where} {len(external)} layer(s) have no catalog algorithm — kept as externalLayers: {', '.join(external)}")
 
         start = excel_date(row["F"]) if row.get("F") else ""
-        end = excel_date(row["G"]) if row.get("G") else start
-        if not row.get("G"):
-            report.append(f"{where} no end date in the sheet — set end = start ({start}).")
+        end = excel_date(row["G"]) if row.get("G") else ""
+        if not end:
+            report.append(f"{where} no end date in the sheet — imported as ongoing (end is optional).")
 
         ev = {
             "id": stac,
             "name": name,
             "stacName": stac,
             "start": start,
-            "end": end,
+            **({"end": end} if end else {}),
             "hazards": hz,
             "locations": [*(gaz.label(i) for i in places), *free],
         }
diff --git a/algorithm-catalog/scripts/rules_parity_test.py b/algorithm-catalog/scripts/rules_parity_test.py
index 958f4aa5..1bf6669e 100644
--- a/algorithm-catalog/scripts/rules_parity_test.py
+++ b/algorithm-catalog/scripts/rules_parity_test.py
@@ -234,6 +234,8 @@ def check_inline_regexes() -> None:
     # --- date ordering -----------------------------------------------------------------------------
     ("date-order", ("2025-01-07", "2025-01-31"), True, None, "start before end"),
     ("date-order", ("2025-01-07", "2025-01-07"), True, None, "single-day event"),
+    ("date-order", ("2026-10-09", ""), True, None, "no end date — the event is ongoing (end is optional)"),
+    ("date-order", ("2026-10-09", "2026-13-01"), False, "format", "an end date, if given, must still be valid"),
     ("date-order", ("2025-01-31", "2025-01-07"), False, "order", "end before start"),
     # --- bbox --------------------------------------------------------------------------------------
     ("bbox", "", True, None, "bbox is optional"),
diff --git a/algorithm-catalog/scripts/validate_data.py b/algorithm-catalog/scripts/validate_data.py
index a553d82d..9b1d9b8f 100644
--- a/algorithm-catalog/scripts/validate_data.py
+++ b/algorithm-catalog/scripts/validate_data.py
@@ -266,16 +266,15 @@ def _is(h: dict[str, Any]) -> bool:
 
 
 def check_dates(start: str, end: str) -> list[Violation]:
-    """ISO shape + ordering. Upstream only checks shape; start<=end is our own catalog invariant."""
+    """ISO shape + ordering. Upstream only checks shape; start<=end is our own catalog invariant.
+    The END date is OPTIONAL: blank means the event is still ongoing. Mirrors rules.ts checkDates."""
     out: list[Violation] = []
     if not start:
         out.append(_err("start", "required", "Start date is required."))
     elif not matches(_ISO_DATE, start):
         out.append(_err("start", "format", f'Start date "{start}" must be YYYY-MM-DD.'))
 
-    if not end:
-        out.append(_err("end", "required", "End date is required."))
-    elif not matches(_ISO_DATE, end):
+    if end and not matches(_ISO_DATE, end):
         out.append(_err("end", "format", f'End date "{end}" must be YYYY-MM-DD.'))
 
     if start and end and matches(_ISO_DATE, start) and matches(_ISO_DATE, end) and start > end:
diff --git a/algorithm-catalog/src/data.ts b/algorithm-catalog/src/data.ts
index 2fedc3d6..8dcdfc2c 100644
--- a/algorithm-catalog/src/data.ts
+++ b/algorithm-catalog/src/data.ts
@@ -242,8 +242,10 @@ export function formatDate(iso: string): string {
   return `${Number(m[3])} ${MONTHS[Number(m[2]) - 1]} ${m[1]}`;
 }
 
-export function formatRange(start: string, end: string): string {
+/** A missing end date means the event is still ongoing. */
+export function formatRange(start: string, end?: string): string {
   if (!start && !end) return "no dates";
+  if (!end) return `${formatDate(start)} – ongoing`;
   if (start === end) return formatDate(start);
   return `${formatDate(start)} – ${formatDate(end)}`;
 }
diff --git a/algorithm-catalog/src/filters/filter.ts b/algorithm-catalog/src/filters/filter.ts
index 5d6abd7d..068d57ea 100644
--- a/algorithm-catalog/src/filters/filter.ts
+++ b/algorithm-catalog/src/filters/filter.ts
@@ -164,7 +164,7 @@ export function applyEvent(f: Filters, u: Universe, ev: ActivationEvent | null):
     eventId: ev.id,
     hazards: new Set(known.length > 0 ? known : u.hazards),
     start: ev.start,
-    end: ev.end,
+    end: ev.end ?? "",
     aoi: ev.locations[0] ?? "",
   };
 }
@@ -229,9 +229,9 @@ export function eventHaystack(
     .toLowerCase();
 }
 
-/** Two windows overlap unless one ends before the other begins. */
-function overlaps(evStart: string, evEnd: string, start: string, end: string): boolean {
-  if (start && evEnd < start) return false;
+/** Two windows overlap unless one ends before the other begins. No end = ongoing, never ended. */
+function overlaps(evStart: string, evEnd: string | undefined, start: string, end: string): boolean {
+  if (start && evEnd && evEnd < start) return false;
   if (end && evStart > end) return false;
   return true;
 }
diff --git a/algorithm-catalog/src/pageExport.ts b/algorithm-catalog/src/pageExport.ts
index ad121849..cad13749 100644
--- a/algorithm-catalog/src/pageExport.ts
+++ b/algorithm-catalog/src/pageExport.ts
@@ -66,7 +66,7 @@ export function pageView(ev: PageEvent, algorithms: Algorithm[], hazards: Hazard
     hazards: ev.hazards.map((h) => hazardLabel(h, hazards)),
     region: ev.page?.region?.trim() ? [ev.page.region.trim()] : ev.locations,
     startDate: ev.start,
-    endDate: ev.end,
+    endDate: ev.end ?? "",
     govLink: ev.page?.govLink ?? "",
     paragraphs: (ev.page?.intro ?? "").split(/\n\s*\n/).map((p) => p.trim()).filter(Boolean),
     stacName: ev.stacName,
diff --git a/algorithm-catalog/src/request/RequestForm.tsx b/algorithm-catalog/src/request/RequestForm.tsx
index 1ecc397d..1b600802 100644
--- a/algorithm-catalog/src/request/RequestForm.tsx
+++ b/algorithm-catalog/src/request/RequestForm.tsx
@@ -97,7 +97,9 @@ function deriveInput(input: AlgorithmInput, d: RequestDraft, tokens: string[]):
     return { input, value: d.start, source: "start date" };
   }
   if (has("end") || n === "date_to" || n === "to_date") {
-    return { input, value: d.end, source: "end date" };
+    return d.end
+      ? { input, value: d.end, source: "end date" }
+      : { input, value: "", source: "no end date — the event is ongoing; set by the operator at run time" };
   }
   if (has("bbox") || has("bound") || has("extent")) {
     return { input, value: d.bbox, source: "bounding box" };
@@ -460,7 +462,9 @@ export default function RequestForm({ algorithms, hazards, events }: Props) {
             <Issues all={violations} field="start" show={shown("start")} />
           </div>
           <div>
-            <label htmlFor="req-end">End date</label>
+            <label htmlFor="req-end">
+              End date <span className="opt">(optional)</span>
+            </label>
             <input
               id="req-end"
               type="date"
@@ -469,6 +473,7 @@ export default function RequestForm({ algorithms, hazards, events }: Props) {
               onChange={(e) => patch({ end: e.target.value })}
               onBlur={() => touch("end")}
             />
+            <div className="req-help">Leave blank while the event is ongoing.</div>
             <Issues all={violations} field="end" show={shown("end")} />
           </div>
         </div>
diff --git a/algorithm-catalog/src/request/drafts.ts b/algorithm-catalog/src/request/drafts.ts
index fdf5c639..3097e3ba 100644
--- a/algorithm-catalog/src/request/drafts.ts
+++ b/algorithm-catalog/src/request/drafts.ts
@@ -23,7 +23,7 @@ export interface RequestDraft {
   name: string; // human-readable, e.g. "California Wildfires January 2025"
   stacName: string; // YYYYMM_Hazard_Location, e.g. "202501_Fire_CA"
   start: string; // YYYY-MM-DD
-  end: string; // YYYY-MM-DD
+  end: string; // YYYY-MM-DD — optional; blank = ongoing
   bbox: string; // "min_lon,min_lat,max_lon,max_lat" — optional
   mgrsTiles: string; // optional, space-separated Sentinel-2 MGRS tiles, e.g. "T17RLN T17RLM"
   wrs2Tiles: string; // optional, space-separated Landsat WRS-2 path/rows, e.g. "171035"
@@ -140,7 +140,7 @@ export function draftToRequest(d: RequestDraft, ts: string): AlgorithmRequest {
       name: d.name.trim(),
       stacName: d.stacName.trim(),
       start: d.start,
-      end: d.end,
+      ...(d.end ? { end: d.end } : {}),
       hazards: [...d.hazards],
       locations: allLocations(d),
     },
@@ -193,7 +193,7 @@ export function requestToDraft(r: AlgorithmRequest): RequestDraft {
   d.name = r.event.name;
   d.stacName = r.event.stacName;
   d.start = r.event.start;
-  d.end = r.event.end;
+  d.end = r.event.end ?? "";
   d.bbox = r.event.bbox ?? "";
   d.mgrsTiles = (r.event.tiles?.mgrs ?? []).join(" ");
   d.wrs2Tiles = (r.event.tiles?.wrs2 ?? []).join(" ");
diff --git a/algorithm-catalog/src/rules.ts b/algorithm-catalog/src/rules.ts
index 9c077e63..ce587f6c 100644
--- a/algorithm-catalog/src/rules.ts
+++ b/algorithm-catalog/src/rules.ts
@@ -214,15 +214,18 @@ export function checkStacName(value: string, hazards: Hazard[] = [], selected: s
   return out;
 }
 
-/** ISO shape + ordering. Upstream only checks shape; start<=end is our own catalog invariant. */
+/**
+ * ISO shape + ordering. Upstream only checks shape; start<=end is our own catalog invariant.
+ * The END date is OPTIONAL: blank means the event is still ongoing (the portal itself records only
+ * a startDate). Only a malformed or out-of-order end is an error.
+ */
 export function checkDates(start: string, end: string): Violation[] {
   const out: Violation[] = [];
   if (!start) out.push(err("start", "required", "Start date is required."));
   else if (!ISO_DATE_RE.test(start))
     out.push(err("start", "format", `Start date "${start}" must be YYYY-MM-DD.`));
 
-  if (!end) out.push(err("end", "required", "End date is required."));
-  else if (!ISO_DATE_RE.test(end)) out.push(err("end", "format", `End date "${end}" must be YYYY-MM-DD.`));
+  if (end && !ISO_DATE_RE.test(end)) out.push(err("end", "format", `End date "${end}" must be YYYY-MM-DD.`));
 
   if (start && end && ISO_DATE_RE.test(start) && ISO_DATE_RE.test(end) && start > end) {
     out.push(err("end", "order", `End date ${end} is before start date ${start}.`));
diff --git a/algorithm-catalog/src/types.ts b/algorithm-catalog/src/types.ts
index bf892254..beed9bb7 100644
--- a/algorithm-catalog/src/types.ts
+++ b/algorithm-catalog/src/types.ts
@@ -135,7 +135,8 @@ export interface ActivationEvent {
   name: string; // human-readable, visible on the website
   stacName: string; // YYYYMM_Hazard_Location
   start: string; // "YYYY-MM-DD"
-  end: string; // "YYYY-MM-DD"
+  /** "YYYY-MM-DD", or absent while the event is ongoing. */
+  end?: string;
   hazards: string[]; // hazard ids
   /**
    * Display strings, shown as the page's "Region". The first `places.length` of them are the
@@ -201,7 +202,7 @@ export interface AlgorithmRequest {
     name: string;
     stacName: string;
     start: string;
-    end: string;
+    end?: string; // absent = ongoing
     hazards: string[];
     locations: string[];
     places?: string[];
diff --git a/docs/ALGORITHM_CATALOG.md b/docs/ALGORITHM_CATALOG.md
index 802c601c..856ad208 100644
--- a/docs/ALGORITHM_CATALOG.md
+++ b/docs/ALGORITHM_CATALOG.md
@@ -77,6 +77,7 @@ hand-kept `hazard_event_info.xlsx`; the Submit form's **2 · Page content** sect
 | US-gov link | "What the U.S. government is doing" | `page.govLink` (https only) |
 | Intro paragraph | Overview | `page.intro` (blank line = paragraph) |
 | Start date | Start Date | `start` |
+| End date | — (the portal records none) | `end` — **optional**; blank = ongoing (shown "– ongoing", never filtered out) |
 | Need/Theme | Need | `page.needs` — the portal's theme **ids** `prepare/respond/recover/resilience` |
 | Hazards | Hazard / categories | `hazards` → `hazards.json` `portal` (its `CONTENT_CATEGORIES`) |
 | Locations | Region | `locations` (+ `places`), or `page.region` override |