Update src/main/java/net/ripe/rpki/ta/util/ValidityPeriods.java

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: lolepezy

src/main/java/net/ripe/rpki/ta/util/ValidityPeriods.java

@@ -10,7 +10,7 @@ public class ValidityPeriods {
 
     // Since this program runs within a script, we can safely assume that all
     // calls to "now" can be replaced with a value calculated only once.
-    private static DateTime globalNow = DateTime.now(DateTimeZone.UTC);
+    private static volatile DateTime globalNow = DateTime.now(DateTimeZone.UTC);
 
     public static DateTime now() {
         return globalNow;