diff --git a/quartz/plugins/transformers/frontmatter.ts b/quartz/plugins/transformers/frontmatter.ts
index ce57428..1103900 100644
--- a/quartz/plugins/transformers/frontmatter.ts
+++ b/quartz/plugins/transformers/frontmatter.ts
@@ -101,7 +101,11 @@ export const FrontMatter: QuartzTransformerPlugin<Partial<Options>> = (userOpts)
             const socialImage = coalesceAliases(data, ["socialImage", "image", "cover"])
 
             const created = coalesceAliases(data, ["created", "date"])
-            if (created) data.created = created
+            if (created) {
+              data.created = created
+              data.modified ||= created // if modified is not set, use created
+            }
+
             const modified = coalesceAliases(data, [
               "modified",
               "lastmod",
diff --git a/quartz/plugins/transformers/lastmod.ts b/quartz/plugins/transformers/lastmod.ts
index 668634a..32a89cc 100644
--- a/quartz/plugins/transformers/lastmod.ts
+++ b/quartz/plugins/transformers/lastmod.ts
@@ -12,7 +12,17 @@ const defaultOptions: Options = {
   priority: ["frontmatter", "git", "filesystem"],
 }
 
+// YYYY-MM-DD
+const iso8601DateOnlyRegex = /^\d{4}-\d{2}-\d{2}$/
+
 function coerceDate(fp: string, d: any): Date {
+  // check ISO8601 date-only format
+  // we treat this one as local midnight as the normal
+  // js date ctor treats YYYY-MM-DD as UTC midnight
+  if (typeof d === "string" && iso8601DateOnlyRegex.test(d)) {
+    d = `${d}T00:00:00`
+  }
+
   const dt = new Date(d)
   const invalidDate = isNaN(dt.getTime()) || dt.getTime() === 0
   if (invalidDate && d !== undefined) {