log periodic per-document statistics about snapshot generation

This is to facilitate alerting to detect if snapshot generation were to
stall for a document.

app/common/normalizedDateTimeString.ts

@@ -0,0 +1,27 @@
+import moment from 'moment';
+
+/**
+ * Output an ISO8601 format datetime string, with timezone.
+ * Any string fed in without timezone is expected to be in UTC.
+ *
+ * When connected to postgres, dates will be extracted as Date objects,
+ * with timezone information. The normalization done here is not
+ * really needed in this case.
+ *
+ * Timestamps in SQLite are stored as UTC, and read as strings
+ * (without timezone information). The normalization here is
+ * pretty important in this case.
+ */
+export function normalizedDateTimeString(dateTime: any): string {
+  if (!dateTime) { return dateTime; }
+  if (dateTime instanceof Date) {
+    return moment(dateTime).toISOString();
+  }
+  if (typeof dateTime === 'string' || typeof dateTime === 'number') {
+    // When SQLite returns a string, it will be in UTC.
+    // Need to make sure it actually have timezone info in it
+    // (will not by default).
+    return moment.utc(dateTime).toISOString();
+  }
+  throw new Error(`normalizedDateTimeString cannot handle ${dateTime}`);
+}