Add default value for date field

packages/standard-components/src/forms/DateTimeField.svelte

@@ -7,12 +7,43 @@
   export let placeholder
   export let disabled = false
   export let enableTime = false
+  export let defaultValue
 
   let fieldState
   let fieldApi
+
+  const parseDate = val => {
+    if (!val) {
+      return null
+    }
+    let date
+    if (isNaN(val)) {
+      // Treat as date string of some sort
+      date = new Date(val)
+    } else {
+      // Treat as numerical timestamp
+      date = new Date(parseInt(val))
+    }
+    const time = date.getTime()
+    if (isNaN(time)) {
+      return null
+    }
+    // By rounding to the nearest second we avoid locking up in an endless
+    // loop in the builder, caused by potentially enriching {{ now }} to every
+    // millisecond.
+    return new Date(Math.floor(time / 1000) * 1000)
+  }
 </script>
 
-<Field {label} {field} {disabled} type="datetime" bind:fieldState bind:fieldApi>
+<Field
+  {label}
+  {field}
+  {disabled}
+  defaultValue={parseDate(defaultValue)}
+  type="datetime"
+  bind:fieldState
+  bind:fieldApi
+>
   {#if fieldState}
     <CoreDatePicker
       value={$fieldState.value}