diff --git a/apps/class-solid/src/components/Analysis.tsx b/apps/class-solid/src/components/Analysis.tsx
index 70c91e9..516b766 100644
--- a/apps/class-solid/src/components/Analysis.tsx
+++ b/apps/class-solid/src/components/Analysis.tsx
@@ -119,7 +119,7 @@ const flatObservations: () => Observation[] = createMemo(() => {
 });
 
 const _allTimes = () =>
-  new Set(flatExperiments().flatMap((e) => e.output?.t ?? []));
+  new Set(flatExperiments().flatMap((e) => e.output?.utcTime ?? []));
 const uniqueTimes = () => [...new Set(_allTimes())].sort((a, b) => a - b);
 
 // TODO: could memoize all reactive elements here, would it make a difference?
@@ -140,9 +140,15 @@ export function TimeSeriesPlot({ analysis }: { analysis: TimeseriesAnalysis }) {
       e.output ? e.output[analysis.yVariable as OutputVariableKey] : [],
     );
 
-  const granularity = () => (analysis.xVariable === "t" ? 600 : undefined);
-  const xLim = () => getNiceAxisLimits(allX(), 0, granularity());
-  const yLim = () => getNiceAxisLimits(allY());
+  const granularities: Record<string, number | undefined> = {
+    t: 600, // 10 minutes in seconds
+    utcTime: 60_000, // 1 minute in milliseconds
+    default: undefined,
+  };
+
+  const roundTo = (v: string) => granularities[v] ?? granularities.default;
+  const xLim = () => getNiceAxisLimits(allX(), 0, roundTo(analysis.xVariable));
+  const yLim = () => getNiceAxisLimits(allY(), 0, roundTo(analysis.yVariable));
 
   const chartData = () =>
     flatExperiments().map((e) => {
@@ -185,11 +191,14 @@ export function TimeSeriesPlot({ analysis }: { analysis: TimeseriesAnalysis }) {
     setResetPlot(analysis.id);
   };
 
-  const formatX = () =>
-    analysis.xVariable === "t" ? formatSeconds : d3.format(".4");
-  const formatY = () =>
-    analysis.yVariable === "t" ? formatSeconds : d3.format(".4");
-
+  // Define axis format
+  const formatters: Record<string, (value: number) => string> = {
+    t: formatSeconds,
+    utcTime: formatUTC,
+    default: d3.format(".4"),
+  };
+  const formatX = () => formatters[analysis.xVariable] ?? formatters.default;
+  const formatY = () => formatters[analysis.yVariable] ?? formatters.default;
   return (
     <>
       {/* TODO: get label for yVariable from model config */}
@@ -264,7 +273,7 @@ export function VerticalProfilePlot({
   const profileData = () =>
     flatExperiments().map((e) => {
       const { config, output, ...formatting } = e;
-      const t = output?.t.indexOf(uniqueTimes()[analysis.time]);
+      const t = output?.utcTime.indexOf(uniqueTimes()[analysis.time]);
       if (config.sw_ml && output && t !== undefined && t !== -1) {
         const outputAtTime = getOutputAtTime(output, t);
         return { ...formatting, data: generateProfiles(config, outputAtTime) };
@@ -407,6 +416,10 @@ function formatSeconds(seconds: number): string {
   return `${hours.toString().padStart(2, "0")}:${minutes.toString().padStart(2, "0")}`;
 }
 
+function formatUTC(milliseconds: number): string {
+  return d3.utcFormat("%H:%M")(new Date(milliseconds));
+}
+
 function TimeSlider(
   time: Accessor<number>,
   timeOptions: Accessor<number[]>,
@@ -429,12 +442,12 @@ function TimeSlider(
       class="w-full max-w-md"
     >
       <div class="flex w-full items-center gap-5">
-        <p>Time: </p>
+        <p class="whitespace-nowrap">Time (UTC): </p>
         <SliderTrack>
           <SliderFill />
           <SliderThumb />
         </SliderTrack>
-        <p>{formatSeconds(timeOptions()[time()])}</p>
+        <p>{formatUTC(timeOptions()[time()])}</p>
       </div>
     </Slider>
   );
@@ -468,7 +481,7 @@ export function ThermodynamicPlot({ analysis }: { analysis: SkewTAnalysis }) {
   const profileData = () =>
     flatExperiments().map((e) => {
       const { config, output, ...formatting } = e;
-      const t = output?.t.indexOf(uniqueTimes()[analysis.time]);
+      const t = output?.utcTime.indexOf(uniqueTimes()[analysis.time]);
       if (config.sw_ml && output && t !== undefined && t !== -1) {
         const outputAtTime = getOutputAtTime(output, t);
         return { ...formatting, data: generateProfiles(config, outputAtTime) };
diff --git a/apps/class-solid/src/lib/store.ts b/apps/class-solid/src/lib/store.ts
index 2e02555..13ff8ba 100644
--- a/apps/class-solid/src/lib/store.ts
+++ b/apps/class-solid/src/lib/store.ts
@@ -279,7 +279,7 @@ export function addAnalysis(name: string) {
         description: "",
         type: "timeseries",
         name: "Timeseries",
-        xVariable: "t",
+        xVariable: "utcTime",
         yVariable: "h",
       } as TimeseriesAnalysis;
       break;
diff --git a/packages/class/src/class.ts b/packages/class/src/class.ts
index 9fd6142..5782516 100644
--- a/packages/class/src/class.ts
+++ b/packages/class/src/class.ts
@@ -270,6 +270,13 @@ export class CLASS {
     return this.ml?.qt || 999;
   }
 
+  get utcTime() {
+    // export time in milliseconds since epoch so bounds calculation and
+    // rendering can happen on app side
+    const t0 = new Date(this._cfg.t0).getTime();
+    return t0 + this.t * 1000;
+  }
+
   /**
    * Retrieve a value by name, treating nested state (wind, ml) as if it's flat.
    *
diff --git a/packages/class/src/config.ts b/packages/class/src/config.ts
index 11a945d..5423eca 100644
--- a/packages/class/src/config.ts
+++ b/packages/class/src/config.ts
@@ -28,6 +28,13 @@ const untypedSchema = {
       default: "",
       "ui:widget": "textarea",
     },
+    // Start date / time in utc
+    t0: {
+      type: "string",
+      title: "Start date and time (ISO 8601)",
+      default: new Date().toISOString(),
+      "ui:group": "Time Control",
+    },
     dt: {
       type: "integer",
       unit: "s",
@@ -63,7 +70,7 @@ const untypedSchema = {
       default: false,
     },
   },
-  required: ["name", "dt", "runtime"],
+  required: ["name", "t0", "dt", "runtime"],
   allOf: [
     {
       if: {
@@ -479,6 +486,7 @@ const untypedSchema = {
 type GeneralConfig = {
   name: string;
   description?: string;
+  t0: string;
   dt: number;
   runtime: number;
 };
diff --git a/packages/class/src/config_utils.test.ts b/packages/class/src/config_utils.test.ts
index d820e09..4495c0a 100644
--- a/packages/class/src/config_utils.test.ts
+++ b/packages/class/src/config_utils.test.ts
@@ -7,6 +7,7 @@ describe("pruneConfig()", () => {
     const preset = {
       name: "Default",
       description: "Default configuration",
+      t0: "2025-09-08T14:30:00Z",
       theta_0: 323,
       h_0: 200,
       dtheta_0: 1,
@@ -26,6 +27,7 @@ describe("pruneConfig()", () => {
     const reference = {
       name: "Higher and Hotter",
       description: "Higher h_0",
+      t0: "2025-09-08T14:30:00Z",
       h_0: 211,
       theta_0: 323,
       dtheta_0: 1,
@@ -45,6 +47,7 @@ describe("pruneConfig()", () => {
     const permutation = {
       name: "Higher",
       description: "",
+      t0: "2025-09-08T14:30:00Z",
       h_0: 222,
       theta_0: 323,
       dtheta_0: 1,
diff --git a/packages/class/src/config_utils.ts b/packages/class/src/config_utils.ts
index 7a6ce86..e5e185b 100644
--- a/packages/class/src/config_utils.ts
+++ b/packages/class/src/config_utils.ts
@@ -73,7 +73,7 @@ export function pruneConfig(
   for (const key in config) {
     const k = key as keyof typeof config;
     const k2 = key as keyof typeof config;
-    if (typeof config[k] === "string") {
+    if (k === "name" || k === "description") {
       // Do not prune name and description
       continue;
     }
diff --git a/packages/class/src/output.ts b/packages/class/src/output.ts
index 7fa5e58..5182789 100644
--- a/packages/class/src/output.ts
+++ b/packages/class/src/output.ts
@@ -10,6 +10,11 @@ export const outputVariables = {
     unit: "s",
     symbol: "t",
   },
+  utcTime: {
+    title: "Time (UTC)",
+    unit: "-",
+    symbol: "UTC",
+  },
   h: {
     title: "ABL height",
     unit: "m",
diff --git a/packages/class/src/runner.ts b/packages/class/src/runner.ts
index 480874a..e3ca529 100644
--- a/packages/class/src/runner.ts
+++ b/packages/class/src/runner.ts
@@ -42,7 +42,7 @@ export function runClass(config: Config, freq = 600): ClassOutput {
   writeOutput();
 
   // Update loop
-  while (model.t < config.runtime) {
+  while (model.t <= config.runtime) {
     model.update();
 
     if (model.t % freq === 0) {
diff --git a/packages/class/src/validate.test.ts b/packages/class/src/validate.test.ts
index 40fcde2..f9dc391 100644
--- a/packages/class/src/validate.test.ts
+++ b/packages/class/src/validate.test.ts
@@ -50,6 +50,7 @@ describe("parse", () => {
     const output = parse(input);
 
     const expected = {
+      t0: output.t0, // dynamic default
       h: 200,
       theta: 288,
       dtheta: 1,