diff --git a/Sources/SotoCognitoAuthenticationSRP/Authenticatable+SRP+async.swift b/Sources/SotoCognitoAuthenticationSRP/Authenticatable+SRP+async.swift
index e6194c9..c9cc8c9 100644
--- a/Sources/SotoCognitoAuthenticationSRP/Authenticatable+SRP+async.swift
+++ b/Sources/SotoCognitoAuthenticationSRP/Authenticatable+SRP+async.swift
@@ -84,7 +84,7 @@ public extension CognitoAuthenticatable {
         dateFormatter.dateFormat = "EEE MMM d HH:mm:ss 'UTC' yyyy"
         dateFormatter.timeZone = TimeZone(identifier: "UTC")
         // cognito expects the dateformat to be in English
-        dateFormatter.locale = Locale(identifier: "en")
+        dateFormatter.locale = Locale(identifier: "en_US_POSIX")
         let timestamp = dateFormatter.string(from: Date())
 
         // construct claim