diff --git a/packages/common/src/i18n/format_date.ts b/packages/common/src/i18n/format_date.ts
index 2e9ba2d7cc4..34a3e3bd2ed 100644
--- a/packages/common/src/i18n/format_date.ts
+++ b/packages/common/src/i18n/format_date.ts
@@ -32,14 +32,14 @@ const NAMED_FORMATS: {[localeId: string]: {[format: string]: string}} = {};
 const DATE_FORMATS_SPLIT =
   /((?:[^BEGHLMOSWYZabcdhmswyz']+)|(?:'(?:[^']|'')*')|(?:G{1,5}|y{1,4}|Y{1,4}|M{1,5}|L{1,5}|w{1,2}|W{1}|d{1,2}|E{1,6}|c{1,6}|a{1,5}|b{1,5}|B{1,5}|h{1,2}|H{1,2}|m{1,2}|s{1,2}|S{1,3}|z{1,4}|Z{1,5}|O{1,4}))([\s\S]*)/;
 
-enum ZoneWidth {
+const enum ZoneWidth {
   Short,
   ShortGMT,
   Long,
   Extended,
 }
 
-enum DateType {
+const enum DateType {
   FullYear,
   Month,
   Date,
@@ -50,7 +50,7 @@ enum DateType {
   Day,
 }
 
-enum TranslationType {
+const enum TranslationType {
   DayPeriods,
   Days,
   Months,