diff --git a/src/timer-service.ts b/src/timer-service.ts
index 98f8da0..ffbe806 100644
--- a/src/timer-service.ts
+++ b/src/timer-service.ts
@@ -85,6 +85,33 @@ export class TimerService {
     return this.state;
   }
 
+  public updateActiveStart(startedAtEpochMs: number): TrackerState {
+    const active = this.state.activeTimer;
+    if (!active) {
+      return this.state;
+    }
+
+    if (!Number.isFinite(startedAtEpochMs)) {
+      throw new Error("Invalid start timestamp.");
+    }
+
+    const now = nowMs();
+    if (startedAtEpochMs > now) {
+      throw new Error("Start must be less than or equal to now.");
+    }
+
+    this.state = {
+      ...this.state,
+      activeTimer: {
+        ...active,
+        startedAtEpochMs
+      }
+    };
+
+    this.persist();
+    return this.state;
+  }
+
   public switchToPage(page: PageRef): TrackerState {
     if (this.state.activeTimer?.pageUuid === page.uuid) {
       return this.state;
diff --git a/src/ui.ts b/src/ui.ts
index 5a00080..d2114ca 100644
--- a/src/ui.ts
+++ b/src/ui.ts
@@ -54,6 +54,8 @@ export class TimetrackerUi {
   private readonly listPages: () => Promise<PageRef[]>;
   private intervalId: number | null = null;
   private editDraft: EditDraft | null = null;
+  private activeStartDraft: string | null = null;
+  private isEditingActiveStart = false;
   private pageOptions: PageRef[] = [];
 
   constructor(
@@ -71,8 +73,8 @@ export class TimetrackerUi {
     this.bindEvents();
     this.render();
     this.intervalId = window.setInterval(() => {
-      // Keep focus stable while an entry is being edited.
-      if (this.editDraft) {
+      // Keep focus stable while any form field is being edited.
+      if (this.editDraft || this.isEditingActiveStart) {
         return;
       }
       this.render();
@@ -90,7 +92,13 @@ export class TimetrackerUi {
     const app = this.ensureContainer();
     const state = this.service.getState();
     const active = state.activeTimer;
-    const activeDuration = active ? Date.now() - active.startedAtEpochMs : 0;
+    const activeDuration = active ? Math.max(0, Date.now() - active.startedAtEpochMs) : 0;
+
+    if (!active && this.activeStartDraft !== null) {
+      this.activeStartDraft = null;
+    }
+
+    const activeStartValue = active ? this.activeStartDraft ?? toIsoLocalInput(active.startedAtEpochMs) : "";
 
     const daySummaries = computeDaySummaries(state.entries);
     const weekSummaries = computeWeekSummaries(state.entries);
@@ -139,6 +147,20 @@ export class TimetrackerUi {
               <button id="tt-stop">Stop active</button>
               <button id="tt-export">Export JSON</button>
             </div>
+            ${active
+              ? `<div class="tt-edit-panel">
+                <strong>Active timer</strong>
+                <div class="tt-edit-grid">
+                  <label>
+                    <span>Start</span>
+                    <input id="tt-active-start" type="datetime-local" value="${this.escapeAttribute(activeStartValue)}" />
+                  </label>
+                </div>
+                <div class="tt-actions">
+                  <button id="tt-active-start-save">Save active start</button>
+                </div>
+              </div>`
+              : ""}
             ${this.editDraft ? this.renderEditPanel() : ""}
             <table>
               <thead>
@@ -250,6 +272,11 @@ export class TimetrackerUi {
         return;
       }
 
+      if (target.id === "tt-active-start-save") {
+        this.saveActiveStartDraft();
+        return;
+      }
+
       if (target.id === "tt-edit-save") {
         this.saveEditDraft();
         return;
@@ -274,7 +301,15 @@ export class TimetrackerUi {
 
     document.addEventListener("input", (event) => {
       const target = event.target as HTMLInputElement | HTMLTextAreaElement | null;
-      if (!target || !this.editDraft) {
+      if (!target) {
+        return;
+      }
+
+      if (target.id === "tt-active-start") {
+        this.activeStartDraft = target.value;
+      }
+
+      if (!this.editDraft) {
         return;
       }
 
@@ -291,6 +326,20 @@ export class TimetrackerUi {
       }
     });
 
+    document.addEventListener("focusin", (event) => {
+      const target = event.target as HTMLElement | null;
+      if (target?.id === "tt-active-start") {
+        this.isEditingActiveStart = true;
+      }
+    });
+
+    document.addEventListener("focusout", (event) => {
+      const target = event.target as HTMLElement | null;
+      if (target?.id === "tt-active-start") {
+        this.isEditingActiveStart = false;
+      }
+    });
+
     document.addEventListener("keydown", (event) => {
       if (event.key === "Escape") {
         if (this.editDraft) {
@@ -377,6 +426,32 @@ export class TimetrackerUi {
     }
   }
 
+  private saveActiveStartDraft(): void {
+    const active = this.service.getState().activeTimer;
+    if (!active) {
+      this.activeStartDraft = null;
+      this.render();
+      return;
+    }
+
+    const rawInput = this.activeStartDraft ?? toIsoLocalInput(active.startedAtEpochMs);
+    const startedAtEpochMs = parseLocalInput(rawInput);
+
+    if (!Number.isFinite(startedAtEpochMs)) {
+      logseq.App.showMsg("Invalid date format.", "error");
+      return;
+    }
+
+    try {
+      this.service.updateActiveStart(startedAtEpochMs);
+      this.activeStartDraft = null;
+      this.render();
+      logseq.App.showMsg("Active start updated.", "success");
+    } catch (error) {
+      logseq.App.showMsg((error as Error).message, "error");
+    }
+  }
+
   private escapeHtml(value: string): string {
     return value
       .replace(/&/g, "&amp;")