Returned date should always in UTC. Timezone is only for display.

tp-date-picker.js

@@ -220,6 +220,7 @@ class TpDatePicker extends FormElement(LitElement) {
       yearsBackwards: { type: Number },
       showYearSelector: { type: Boolean },
       events: { type: Array },
+      timeZone: { type: String }, // User's timezone for date interpretation
     };
   }
 
@@ -289,7 +290,16 @@ class TpDatePicker extends FormElement(LitElement) {
     for (const el of e.composedPath()) {
       if (el.date !== undefined) {
         this.value = el.date;
-        this.dispatchEvent(new CustomEvent('value-changed', { detail: this.value, bubbles: true, composed: true }));
+        // Interpret the selected date as being in the user's timezone, output as UTC ISO string
+        const selectedDate = el.date;
+        let outputValue;
+        const zone = this.timeZone || 'local';
+        const localDate = DateTime.fromObject(
+          { year: selectedDate.year, month: selectedDate.month, day: selectedDate.day },
+          { zone }
+        );
+        outputValue = localDate.toUTC().toISO();
+        this.dispatchEvent(new CustomEvent('value-changed', { detail: outputValue, bubbles: true, composed: true }));
         return;
       }
     }