diff --git a/pgpainless-core/src/main/java/org/pgpainless/signature/subpackets/SignatureSubpacketGeneratorUtil.java b/pgpainless-core/src/main/java/org/pgpainless/signature/subpackets/SignatureSubpacketGeneratorUtil.java
index fe493ac7..eeb87a10 100644
--- a/pgpainless-core/src/main/java/org/pgpainless/signature/subpackets/SignatureSubpacketGeneratorUtil.java
+++ b/pgpainless-core/src/main/java/org/pgpainless/signature/subpackets/SignatureSubpacketGeneratorUtil.java
@@ -92,28 +92,10 @@ public final class SignatureSubpacketGeneratorUtil {
                                                                 @Nonnull Date creationDate,
                                                                 PGPSignatureSubpacketGenerator subpacketGenerator) {
         removeAllPacketsOfType(SignatureSubpacketTags.KEY_EXPIRE_TIME, subpacketGenerator);
-        long secondsToExpire = getKeyLifetimeInSeconds(expirationDate, creationDate);
+        long secondsToExpire = SignatureSubpacketsUtil.getKeyLifetimeInSeconds(expirationDate, creationDate);
         subpacketGenerator.setKeyExpirationTime(true, secondsToExpire);
     }
 
-    /**
-     * Calculate the duration in seconds until the key expires after creation.
-     *
-     * @param expirationDate new expiration date
-     * @param creationTime key creation time
-     * @return life time of the key in seconds
-     */
-    private static long getKeyLifetimeInSeconds(Date expirationDate, @Nonnull Date creationTime) {
-        long secondsToExpire = 0; // 0 means "no expiration"
-        if (expirationDate != null) {
-            if (creationTime.after(expirationDate)) {
-                throw new IllegalArgumentException("Key MUST NOT expire before being created. (creation: " + creationTime + ", expiration: " + expirationDate + ")");
-            }
-            secondsToExpire = (expirationDate.getTime() - creationTime.getTime()) / 1000;
-        }
-        return secondsToExpire;
-    }
-
     /**
      * Return true, if the subpacket generator has a {@link KeyFlags} subpacket which carries the given key flag.
      * Returns false, if no {@link KeyFlags} subpacket is present.
diff --git a/pgpainless-core/src/main/java/org/pgpainless/signature/subpackets/SignatureSubpacketsUtil.java b/pgpainless-core/src/main/java/org/pgpainless/signature/subpackets/SignatureSubpacketsUtil.java
index 751c9f73..d564bfce 100644
--- a/pgpainless-core/src/main/java/org/pgpainless/signature/subpackets/SignatureSubpacketsUtil.java
+++ b/pgpainless-core/src/main/java/org/pgpainless/signature/subpackets/SignatureSubpacketsUtil.java
@@ -11,6 +11,7 @@ import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
 
+import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 
 import org.bouncycastle.bcpg.sig.Exportable;
@@ -204,6 +205,25 @@ public final class SignatureSubpacketsUtil {
         return SignatureUtils.datePlusSeconds(signingKey.getCreationTime(), subpacket.getTime());
     }
 
+    /**
+     * Calculate the duration in seconds until the key expires after creation.
+     *
+     * @param expirationDate new expiration date
+     * @param creationDate key creation time
+     * @return life time of the key in seconds
+     */
+    public static long getKeyLifetimeInSeconds(@Nullable Date expirationDate, @Nonnull Date creationDate) {
+        long secondsToExpire = 0; // 0 means "no expiration"
+        if (expirationDate != null) {
+            if (creationDate.after(expirationDate)) {
+                throw new IllegalArgumentException("Key MUST NOT expire before being created. " +
+                        "(creation: " + creationDate + ", expiration: " + expirationDate + ")");
+            }
+            secondsToExpire = (expirationDate.getTime() - creationDate.getTime()) / 1000;
+        }
+        return secondsToExpire;
+    }
+
     /**
      * Return the revocable subpacket of this signature.
      * We only look for it in the hashed area of the signature.