https://github.com/mailersend/mailersend-java/blob/021ee36bb653103f9a4e0b0482b87624fed4a579/src/main/java/com/mailersend/sdk/emails/Emails.java#L111 There we need this code: email.sendAtStamp = (email.sendAt != null) ? String.valueOf(email.sendAt.getTime() / 1000) : null;