diff --git a/schemas/root.schema.json b/schemas/root.schema.json
index 0a21bed..99368a4 100644
--- a/schemas/root.schema.json
+++ b/schemas/root.schema.json
@@ -37,7 +37,7 @@
         },
         "expires": {
           "type": "string",
-          "format": "date-time"
+          "pattern": "^[0-9]{4}-[0-9]{2}-[0-9]{2}T[0-9]{2}:[0-9]{2}:[0-9]{2}Z$"
         },
         "keys": {
           "type": "object",
diff --git a/schemas/snapshot.schema.json b/schemas/snapshot.schema.json
index 91db7d6..ec7f0cd 100644
--- a/schemas/snapshot.schema.json
+++ b/schemas/snapshot.schema.json
@@ -37,7 +37,7 @@
         },
         "expires": {
           "type": "string",
-          "format": "date-time"
+          "pattern": "^[0-9]{4}-[0-9]{2}-[0-9]{2}T[0-9]{2}:[0-9]{2}:[0-9]{2}Z$"
         },
         "meta": {
           "type": "object",
diff --git a/schemas/targets.schema.json b/schemas/targets.schema.json
index 429e568..8828616 100644
--- a/schemas/targets.schema.json
+++ b/schemas/targets.schema.json
@@ -110,7 +110,7 @@
         },
         "expires": {
           "type": "string",
-          "format": "date-time"
+          "pattern": "^[0-9]{4}-[0-9]{2}-[0-9]{2}T[0-9]{2}:[0-9]{2}:[0-9]{2}Z$"
         },
         "spec_version": {
           "type": "string",
diff --git a/schemas/timestamp.schema.json b/schemas/timestamp.schema.json
index 963bb77..7282c9a 100644
--- a/schemas/timestamp.schema.json
+++ b/schemas/timestamp.schema.json
@@ -37,7 +37,7 @@
         },
         "expires": {
           "type": "string",
-          "format": "date-time"
+          "pattern": "^[0-9]{4}-[0-9]{2}-[0-9]{2}T[0-9]{2}:[0-9]{2}:[0-9]{2}Z$"
         },
         "meta": {
           "type": "object",