diff --git a/assets/uriahtalks-horizon-explorer.js b/assets/uriahtalks-horizon-explorer.js
new file mode 100644
index 00000000..575f47e9
--- /dev/null
+++ b/assets/uriahtalks-horizon-explorer.js
@@ -0,0 +1,152 @@
+// Great Docs-compatible renderer for the uriahtalks horizon explorer.
+class UriahtalksHorizonExplorer extends HTMLElement {
+  connectedCallback() {
+    if (this.dataset.ready === "true") return;
+    this.dataset.ready = "true";
+
+    const competingAsCensored =
+      this.hasAttribute("competing-as-censored");
+    const min = Number(this.getAttribute("min") || 5);
+    const max = Number(this.getAttribute("max") || 50);
+    const step = Number(this.getAttribute("step") || 5);
+    const initial = Number(this.getAttribute("horizon") || 30);
+    const title = this.getAttribute("heading") || "Explore the horizon";
+    const accent = this.getAttribute("accent-color") || "#7a9a01";
+
+    this.style.setProperty("--uriah-horizon-accent", accent);
+    this.innerHTML = `
+      <div class="uriah-horizon-heading">${title}</div>
+      <label class="uriah-horizon-control">
+        <span>Fixed time horizon: <output>${initial}</output></span>
+        <input type="range" min="${min}" max="${max}" step="${step}"
+          value="${initial}" aria-label="Fixed time horizon">
+      </label>
+      <div class="uriah-horizon-chart" role="img"
+        aria-label="Follow-up timelines classified at the selected horizon"></div>
+      <div class="uriah-horizon-key"></div>
+    `;
+
+    const observations = [
+      [1, .482, "primary"], [2, .194, "primary"],
+      [3, .998, "primary"], [4, .372, "primary"],
+      [5, .696, "none"], [6, .284, "competing"],
+      [7, .784, "primary"], [8, .920, "competing"],
+      [9, .630, "none"], [10, .086, "primary"],
+    ].map(([id, fraction, outcome]) => ({
+      id, outcome, time: fraction * max,
+    }));
+
+    const states = {
+      primary: ["🤢", "Primary event"],
+      competing: ["💀", "Competing event"],
+      censored: ["🤬", "Censored"],
+      nonEvent: ["🤨", "Non-event through horizon"],
+    };
+    const input = this.querySelector("input");
+    const output = this.querySelector("output");
+    const chart = this.querySelector(".uriah-horizon-chart");
+    const key = this.querySelector(".uriah-horizon-key");
+    const ns = "http://www.w3.org/2000/svg";
+
+    const classify = (observation, horizon) => {
+      if (observation.time > horizon) return states.nonEvent;
+      if (observation.outcome === "primary") return states.primary;
+      if (observation.outcome === "competing") {
+        return competingAsCensored ? states.censored : states.competing;
+      }
+      return states.censored;
+    };
+
+    const render = () => {
+      const horizon = Number(input.value);
+      output.value = horizon;
+      chart.replaceChildren();
+      const width = 900, height = 390;
+      const margin = { top: 22, right: 28, bottom: 48, left: 58 };
+      const innerWidth = width - margin.left - margin.right;
+      const rowHeight =
+        (height - margin.top - margin.bottom) / observations.length;
+      const x = value => margin.left + (value / max) * innerWidth;
+      const svg = document.createElementNS(ns, "svg");
+      svg.setAttribute("viewBox", `0 0 ${width} ${height}`);
+      svg.setAttribute("class", "uriah-horizon-svg");
+
+      const tickStep = Math.max(step, max / 5);
+      for (let tick = 0; tick <= max; tick += tickStep) {
+        const line = document.createElementNS(ns, "line");
+        line.setAttribute("x1", x(tick));
+        line.setAttribute("x2", x(tick));
+        line.setAttribute("y1", height - margin.bottom);
+        line.setAttribute("y2", height - margin.bottom + 6);
+        line.setAttribute("class", "uriah-axis");
+        svg.appendChild(line);
+        const text = document.createElementNS(ns, "text");
+        text.setAttribute("x", x(tick));
+        text.setAttribute("y", height - margin.bottom + 24);
+        text.setAttribute("class", "uriah-tick");
+        text.textContent = Number(tick.toFixed(2));
+        svg.appendChild(text);
+      }
+
+      const axisLabel = document.createElementNS(ns, "text");
+      axisLabel.setAttribute("x", margin.left + innerWidth / 2);
+      axisLabel.setAttribute("y", height - 5);
+      axisLabel.setAttribute("class", "uriah-axis-label");
+      axisLabel.textContent = "Follow-up time";
+      svg.appendChild(axisLabel);
+
+      const horizonLine = document.createElementNS(ns, "line");
+      horizonLine.setAttribute("x1", x(horizon));
+      horizonLine.setAttribute("x2", x(horizon));
+      horizonLine.setAttribute("y1", margin.top - 8);
+      horizonLine.setAttribute("y2", height - margin.bottom);
+      horizonLine.setAttribute("class", "uriah-horizon-line");
+      svg.appendChild(horizonLine);
+
+      observations.forEach((observation, index) => {
+        const y = margin.top + rowHeight * (index + .5);
+        const displayTime = Math.min(observation.time, horizon);
+        const state = classify(observation, horizon);
+        const label = document.createElementNS(ns, "text");
+        label.setAttribute("x", margin.left - 14);
+        label.setAttribute("y", y);
+        label.setAttribute("class", "uriah-row-label");
+        label.textContent = observation.id;
+        svg.appendChild(label);
+        const followup = document.createElementNS(ns, "line");
+        followup.setAttribute("x1", x(0));
+        followup.setAttribute("x2", x(displayTime));
+        followup.setAttribute("y1", y);
+        followup.setAttribute("y2", y);
+        followup.setAttribute("class", "uriah-followup-line");
+        svg.appendChild(followup);
+        const marker = document.createElementNS(ns, "text");
+        marker.setAttribute("x", x(displayTime));
+        marker.setAttribute("y", y);
+        marker.setAttribute("class", "uriah-emoji-marker");
+        marker.textContent = state[0];
+        const tooltip = document.createElementNS(ns, "title");
+        tooltip.textContent =
+          `Observation ${observation.id}: ${state[1]}; observed time ${observation.time.toFixed(2)}`;
+        marker.appendChild(tooltip);
+        svg.appendChild(marker);
+      });
+
+      chart.appendChild(svg);
+      const active = competingAsCensored
+        ? [states.primary, states.censored, states.nonEvent]
+        : [states.primary, states.competing, states.censored, states.nonEvent];
+      key.replaceChildren(...active.map(state => {
+        const item = document.createElement("span");
+        item.innerHTML = `<span aria-hidden="true">${state[0]}</span> ${state[1]}`;
+        return item;
+      }));
+    };
+
+    input.addEventListener("input", render);
+    render();
+  }
+}
+
+customElements.define("uriahtalks-horizon-explorer", UriahtalksHorizonExplorer);
+
diff --git a/great-docs.yml b/great-docs.yml
index 2c40814f..a8293201 100644
--- a/great-docs.yml
+++ b/great-docs.yml
@@ -14,7 +14,18 @@ include_in_header:
       <script>
       document.addEventListener('DOMContentLoaded', () => {
         const nav = document.querySelector('nav.navbar ul.navbar-nav.me-auto');
-        if (!nav || nav.querySelector('[data-rtichoke-blog-link]')) return;
+        if (!nav || nav.querySelector('[data-rtichoke-guides-link]')) return;
+
+        const guidesItem = document.createElement('li');
+        guidesItem.className = 'nav-item';
+        guidesItem.setAttribute('data-rtichoke-guides-link', '');
+
+        const guidesLink = document.createElement('a');
+        guidesLink.className = 'nav-link';
+        const offset = document.querySelector('meta[name="quarto:offset"]');
+        guidesLink.href = (offset ? offset.content : '') + 'index.html';
+        guidesLink.textContent = 'Guides';
+        guidesItem.appendChild(guidesLink);
 
         const item = document.createElement('li');
         item.className = 'nav-item';
@@ -29,9 +40,20 @@ include_in_header:
         const reference = [...nav.children].find((el) =>
           el.textContent.trim().toLowerCase().includes('reference')
         );
+        nav.insertBefore(guidesItem, reference || null);
         nav.insertBefore(item, reference || null);
       });
       </script>
+  - text: |
+      <script>
+      (function () {
+        var script = document.createElement('script');
+        var offset = document.querySelector('meta[name="quarto:offset"]');
+        script.src = (offset ? offset.content : '') + 'assets/uriahtalks-horizon-explorer.js';
+        script.defer = true;
+        document.head.appendChild(script);
+      })();
+      </script>
 
 site:
   css: site.css
@@ -85,3 +107,4 @@ reference:
       - create_decision_curve
       - create_decision_curve_times
       - plot_decision_curve
+
diff --git a/site.css b/site.css
index 14644969..29d4039f 100644
--- a/site.css
+++ b/site.css
@@ -182,3 +182,94 @@ footer.footer {
     --rtichoke-shadow: none;
   }
 }
+uriahtalks-horizon-explorer {
+  color: inherit;
+  background: transparent;
+  container-type: inline-size;
+  display: block;
+  margin: 1rem 0 1.5rem;
+}
+
+uriahtalks-horizon-explorer .uriah-horizon-heading {
+  font-size: 1.1em;
+  font-weight: 650;
+  margin-bottom: .65rem;
+}
+
+uriahtalks-horizon-explorer .uriah-horizon-control {
+  display: grid;
+  gap: .3rem;
+  max-width: 34rem;
+}
+
+uriahtalks-horizon-explorer input[type="range"] {
+  accent-color: var(--uriah-horizon-accent);
+  width: 100%;
+}
+
+uriahtalks-horizon-explorer .uriah-horizon-chart {
+  background: transparent;
+  overflow-x: auto;
+  width: 100%;
+}
+
+uriahtalks-horizon-explorer .uriah-horizon-svg {
+  background: transparent;
+  display: block;
+  min-width: 620px;
+  width: 100%;
+}
+
+uriahtalks-horizon-explorer .uriah-axis {
+  stroke: currentColor;
+  stroke-width: 1;
+}
+
+uriahtalks-horizon-explorer .uriah-tick,
+uriahtalks-horizon-explorer .uriah-axis-label,
+uriahtalks-horizon-explorer .uriah-row-label {
+  fill: currentColor;
+  font: 14px Commissioner, system-ui, sans-serif;
+}
+
+uriahtalks-horizon-explorer .uriah-tick,
+uriahtalks-horizon-explorer .uriah-axis-label {
+  text-anchor: middle;
+}
+
+uriahtalks-horizon-explorer .uriah-row-label {
+  dominant-baseline: middle;
+  text-anchor: end;
+}
+
+uriahtalks-horizon-explorer .uriah-followup-line {
+  stroke: color-mix(in srgb, currentColor 38%, transparent);
+  stroke-width: 2;
+}
+
+uriahtalks-horizon-explorer .uriah-horizon-line {
+  stroke: var(--uriah-horizon-accent);
+  stroke-dasharray: 7 6;
+  stroke-width: 4;
+}
+
+uriahtalks-horizon-explorer .uriah-emoji-marker {
+  cursor: help;
+  dominant-baseline: central;
+  font: 30px "Segoe UI Emoji", "Apple Color Emoji", sans-serif;
+  text-anchor: middle;
+}
+
+uriahtalks-horizon-explorer .uriah-horizon-key {
+  display: flex;
+  flex-wrap: wrap;
+  font-size: .9em;
+  gap: .45rem 1.1rem;
+}
+
+@media (max-width: 650px) {
+  uriahtalks-horizon-explorer .uriah-horizon-svg {
+    width: 720px;
+  }
+}
+
diff --git a/user_guide/02-fixed-time-horizons.qmd b/user_guide/02-fixed-time-horizons.qmd
new file mode 100644
index 00000000..1268bd5e
--- /dev/null
+++ b/user_guide/02-fixed-time-horizons.qmd
@@ -0,0 +1,54 @@
+---
+title: "Fixed Time Horizons"
+guide-section: "Before You Validate"
+---
+
+A time-to-event prediction must specify when the outcome is evaluated. Use
+`fixed_time_horizons` to declare those times in the same unit as `times` and
+the prediction model:
+
+```python
+fixed_time_horizons = [5.0, 10.0]  # years
+```
+
+Choose clinically meaningful horizons before inspecting performance. Changing
+the horizon changes the outcome being validated.
+
+## Update administrative censoring
+
+At a fixed horizon:
+
+- an event observed after the horizon is a 🤨 non-event *at that horizon*;
+- event-free follow-up ending before the horizon is 🤬 censored;
+- a primary event observed by the horizon remains 🤢; and
+- a competing event observed by the horizon remains 💀.
+
+## Explore the horizon
+
+Move the slider to see how the selected horizon changes each observation. The
+vertical line marks the horizon; information after it is not used.
+
+<uriahtalks-horizon-explorer
+  min="10" max="50" step="10" horizon="50"
+  accent-color="#7a9a01"
+  heading="Explore the horizon">
+</uriahtalks-horizon-explorer>
+
+The symbols are:
+
+- 🤬 censored before the horizon;
+- 🤢 primary event;
+- 🤨 non-event through the horizon; and
+- 💀 competing event.
+
+## Pass horizons to `rtichoke`
+
+```python
+performance_data = rk.prepare_performance_data_times(
+    probs=probs,
+    reals=reals,
+    times=times,
+    fixed_time_horizons=[5.0, 10.0],
+)
+```
+