diff --git a/client/html/post_readonly_sidebar.tpl b/client/html/post_readonly_sidebar.tpl
index 7be5ba3e..d3f88c50 100644
--- a/client/html/post_readonly_sidebar.tpl
+++ b/client/html/post_readonly_sidebar.tpl
@@ -26,6 +26,11 @@
             <%= ctx.makeRelativeTime(ctx.post.creationTime) %>
         </section>
 
+        <section class='file-last-modified-time'>
+            <i class='fa fa-clock-o'></i>Source last modified
+            <%= ctx.makeRelativeTime(ctx.post.fileLastModifiedTime) %>
+        </section>
+
         <% if (ctx.enableSafety) { %>
             <section class='safety'>
                 <i class='fa fa-circle safety-<%- ctx.post.safety %>'></i><!--
diff --git a/client/js/controllers/post_upload_controller.js b/client/js/controllers/post_upload_controller.js
index d317be59..2ad02afc 100644
--- a/client/js/controllers/post_upload_controller.js
+++ b/client/js/controllers/post_upload_controller.js
@@ -177,6 +177,7 @@ class PostUploadController {
         if (uploadable.url) {
             post.source = uploadable.url;
         }
+        post.fileLastModifiedTime = uploadable.file.lastModified; // The client already reads the data anyway, which is convenient
         return post;
     }
 }
diff --git a/client/js/models/post.js b/client/js/models/post.js
index e90d0b26..59b34189 100644
--- a/client/js/models/post.js
+++ b/client/js/models/post.js
@@ -42,6 +42,10 @@ class Post extends events.EventTarget {
         return this._creationTime;
     }
 
+    get fileLastModifiedTime() {
+        return this._fileLastModifiedTime;
+    }
+
     get user() {
         return this._user;
     }
@@ -142,6 +146,10 @@ class Post extends events.EventTarget {
         return this._hasCustomThumbnail;
     }
 
+    set fileLastModifiedTime(value) {
+        this._fileLastModifiedTime = value;
+    }
+
     set flags(value) {
         this._flags = value;
     }
@@ -269,6 +277,9 @@ class Post extends events.EventTarget {
         if (this._source !== this._orig._source) {
             detail.source = this._source;
         }
+        if (this._fileLastModifiedTime !== this._orig._fileLastModifiedTime) {
+            detail.fileLastModifiedTime = this._fileLastModifiedTime;
+        }
 
         let apiPromise = this._id
             ? api.put(uri.formatApiLink("post", this.id), detail, files)
@@ -460,6 +471,7 @@ class Post extends events.EventTarget {
             _type: response.type,
             _mimeType: response.mimeType,
             _creationTime: response.creationTime,
+            _fileLastModifiedTime: response.fileLastModifiedTime,
             _user: response.user,
             _safety: response.safety,
             _contentUrl: response.contentUrl,
diff --git a/client/js/views/post_upload_view.js b/client/js/views/post_upload_view.js
index 7daf4fb0..0610e57b 100644
--- a/client/js/views/post_upload_view.js
+++ b/client/js/views/post_upload_view.js
@@ -82,6 +82,10 @@ class File extends Uploadable {
         return this.file.type;
     }
 
+    get fileLastModifiedTime() {
+        return this.file.fileLastModifiedTime;
+    }
+
     get previewUrl() {
         return this._previewUrl;
     }
diff --git a/server/szurubooru/api/post_api.py b/server/szurubooru/api/post_api.py
index bf40bc07..9262af56 100644
--- a/server/szurubooru/api/post_api.py
+++ b/server/szurubooru/api/post_api.py
@@ -1,3 +1,4 @@
+import time
 from datetime import datetime
 from typing import Dict, List, Optional
 
@@ -86,6 +87,10 @@ def create_post(
     posts.update_post_relations(post, relations)
     posts.update_post_notes(post, notes)
     posts.update_post_flags(post, flags)
+    if ctx.has_param("fileLastModifiedTime"):
+        posts.update_post_file_last_modified_time(
+            post, ctx.get_as_param("file_last_modified_time")
+        )
     if ctx.has_file("thumbnail"):
         posts.update_post_thumbnail(post, ctx.get_file("thumbnail"))
     ctx.session.add(post)
diff --git a/server/szurubooru/func/posts.py b/server/szurubooru/func/posts.py
index 67996b64..2816bb52 100644
--- a/server/szurubooru/func/posts.py
+++ b/server/szurubooru/func/posts.py
@@ -169,6 +169,7 @@ class PostSerializer(serialization.BaseSerializer):
             "version": self.serialize_version,
             "creationTime": self.serialize_creation_time,
             "lastEditTime": self.serialize_last_edit_time,
+            "fileLastModifiedTime": self.serialize_file_last_modified_time,
             "safety": self.serialize_safety,
             "source": self.serialize_source,
             "type": self.serialize_type,
@@ -212,6 +213,9 @@ class PostSerializer(serialization.BaseSerializer):
     def serialize_last_edit_time(self) -> Any:
         return self.post.last_edit_time
 
+    def serialize_file_last_modified_time(self) -> Any:
+        return self.post.file_last_modified_time
+
     def serialize_safety(self) -> Any:
         return SAFETY_MAP[self.post.safety]
 
@@ -408,6 +412,7 @@ def create_post(
     post.safety = model.Post.SAFETY_SAFE
     post.user = user
     post.creation_time = datetime.utcnow()
+    post.file_last_modified_time = datetime.utcnow()
     post.flags = []
 
     post.type = ""
@@ -421,6 +426,15 @@ def create_post(
     return post, new_tags
 
 
+def update_post_file_last_modified_time(
+    post: model.Post, timestamp: int
+) -> None:
+    assert post
+
+    # Timestamp is in ms, so it must be divided by 1000 otherwise out of range
+    post.file_last_modified_time = datetime.fromtimestamp(timestamp / 1000)
+
+
 def update_post_safety(post: model.Post, safety: str) -> None:
     assert post
     safety = util.flip(SAFETY_MAP).get(safety, None)
diff --git a/server/szurubooru/migrations/versions/336a76ec1338_create_post_tables.py b/server/szurubooru/migrations/versions/336a76ec1338_create_post_tables.py
index aea6cc78..69fcbfc9 100644
--- a/server/szurubooru/migrations/versions/336a76ec1338_create_post_tables.py
+++ b/server/szurubooru/migrations/versions/336a76ec1338_create_post_tables.py
@@ -20,6 +20,7 @@ def upgrade():
         sa.Column("id", sa.Integer(), nullable=False),
         sa.Column("user_id", sa.Integer(), nullable=True),
         sa.Column("creation_time", sa.DateTime(), nullable=False),
+        sa.Column("file_last_modified_time", sa.DateTime(), nullable=False),
         sa.Column("last_edit_time", sa.DateTime(), nullable=True),
         sa.Column("safety", sa.Unicode(length=32), nullable=False),
         sa.Column("type", sa.Unicode(length=32), nullable=False),
diff --git a/server/szurubooru/model/post.py b/server/szurubooru/model/post.py
index 9e06a743..4cbb5e29 100644
--- a/server/szurubooru/model/post.py
+++ b/server/szurubooru/model/post.py
@@ -210,6 +210,7 @@ class Post(Base):
     version = sa.Column("version", sa.Integer, default=1, nullable=False)
     creation_time = sa.Column("creation_time", sa.DateTime, nullable=False)
     last_edit_time = sa.Column("last_edit_time", sa.DateTime)
+    file_last_modified_time = sa.Column("file_last_modified_time", sa.DateTime, nullable=False)
     safety = sa.Column("safety", sa.Unicode(32), nullable=False)
     source = sa.Column("source", sa.Unicode(2048))
     flags_string = sa.Column("flags", sa.Unicode(32), default="")
diff --git a/server/szurubooru/tests/conftest.py b/server/szurubooru/tests/conftest.py
index 6e127bcd..213f1134 100644
--- a/server/szurubooru/tests/conftest.py
+++ b/server/szurubooru/tests/conftest.py
@@ -168,6 +168,7 @@ def post_factory():
         post.flags = []
         post.mime_type = "application/octet-stream"
         post.creation_time = datetime(1996, 1, 1)
+        post.file_last_modified_time = datetime(1997, 1, 1)
         return post
 
     return factory
diff --git a/server/szurubooru/tests/func/test_posts.py b/server/szurubooru/tests/func/test_posts.py
index acedf6f3..3f3116d1 100644
--- a/server/szurubooru/tests/func/test_posts.py
+++ b/server/szurubooru/tests/func/test_posts.py
@@ -122,6 +122,7 @@ def test_serialize_post(
         post.post_id = 1
         post.creation_time = datetime(1997, 1, 1)
         post.last_edit_time = datetime(1998, 1, 1)
+        post.file_last_modified_time = datetime(1999, 1, 1)
         post.tags = [
             tag_factory(
                 names=["tag1", "tag2"],
@@ -215,6 +216,7 @@ def test_serialize_post(
             "version": 1,
             "creationTime": datetime(1997, 1, 1),
             "lastEditTime": datetime(1998, 1, 1),
+            "fileLastModifiedTime": datetime(1999, 1, 1),
             "safety": "safe",
             "source": "4gag",
             "type": "image",
diff --git a/server/szurubooru/tests/model/test_post.py b/server/szurubooru/tests/model/test_post.py
index f01455d3..6c6c2f9f 100644
--- a/server/szurubooru/tests/model/test_post.py
+++ b/server/szurubooru/tests/model/test_post.py
@@ -24,6 +24,7 @@ def test_saving_post(post_factory, user_factory, tag_factory):
     post.checksum = "deadbeef"
     post.creation_time = datetime(1997, 1, 1)
     post.last_edit_time = datetime(1998, 1, 1)
+    post.file_last_modified_time = datetime(1999, 1, 1)
     post.mime_type = "application/whatever"
     db.session.add_all([user, tag1, tag2, related_post1, related_post2, post])
 
@@ -44,6 +45,7 @@ def test_saving_post(post_factory, user_factory, tag_factory):
     assert post.checksum == "deadbeef"
     assert post.creation_time == datetime(1997, 1, 1)
     assert post.last_edit_time == datetime(1998, 1, 1)
+    assert post.file_last_modified_time == datetime(1999, 1, 1)
     assert len(post.relations) == 2
     # relation bidirectionality is realized on business level in func.posts
     assert len(related_post1.relations) == 0