diff --git a/README.md b/README.md
index a18075b..da29cb8 100644
--- a/README.md
+++ b/README.md
@@ -20,4 +20,8 @@ using similar syntax for something else, you may need to change
 the `separator` setting in `zasd.conf.py`.
 
 **DISCLAIMER:** This program is still in beta. Use at your own
-risk. It works fine on my system, but your mileage may vary.
\ No newline at end of file
+risk. It works fine on my system, but your mileage may vary.
+
+## Requirements
+
+* `apscheduler`
diff --git a/src/zasd.py b/src/zasd.py
index 39d386e..f3ea775 100644
--- a/src/zasd.py
+++ b/src/zasd.py
@@ -199,7 +199,7 @@ async def snapshot_creation_task(schedule, fs):
     serial = make_snapshot_serial()
     recursive = schedule['recursive']
 
-    if get_fs_flag(fs, tag):
+    if not schedule['if_modified'] or get_fs_flag(fs, tag):
         # Clear tag-modified flags for this tag on filesystem
         clear_fs_flag(fs, tag)
 
@@ -239,13 +239,21 @@ def snapshot_matches_tag(snapshot, tag):
 
 # Get tag of snapshot
 def get_snapshot_tag(snapshot):
-    (tag, serial) = get_snapshot_fields(snapshot)
-    return tag
+    fields = get_snapshot_fields(snapshot)
+    if len(fields) == 2:
+        (tag, serial) = fields
+        return tag
+    else:
+        return ''
 
 # Get serial number of snapshot
 def get_snapshot_serial(snapshot):
-    (tag, serial) = get_snapshot_fields(snapshot)
-    return serial
+    fields = get_snapshot_fields(snapshot)
+    if len(fields) == 2:
+        (tag, serial) = fields
+        return serial
+    else:
+        return ''
 
 # Get tuple of fields in snapshot name
 def get_snapshot_fields(snapshot):
@@ -302,18 +310,24 @@ def load_snapshot_schedules():
 
 async def main_task():
     global config, event_loop, scheduler, fs_listeners
-
     
     scheduler = AsyncIOPriorityScheduler(
             event_loop = event_loop,
             executors  = {'default': AsyncIOPriorityExecutor()})
  
+    monitor = False
+    for schedule in get_schedules():
+        if schedule['if_modified']:
+            monitor = True
+
     # Watch file system mountpoints
-    fs_listeners = dict()
-    for fs in zfs_get_filesystems():
-        await event_loop.subprocess_exec(
-                lambda: FSWatchProtocol(fs), config['fswatch_path'], '-o', fs['mountpoint'], stdout=PIPE)
- 
+    if monitor:
+        logger.info('Starting file system watcher')
+        fs_listeners = dict()
+        for fs in zfs_get_filesystems():
+            await event_loop.subprocess_exec(
+                    lambda: FSWatchProtocol(fs), config['fswatch_path'], '-o', fs['mountpoint'], stdout=PIPE)
+     
     load_snapshot_schedules()
     
     scheduler.start()
diff --git a/src/zasd/config.py b/src/zasd/config.py
index 332aa01..1a21f8a 100644
--- a/src/zasd/config.py
+++ b/src/zasd/config.py
@@ -44,6 +44,7 @@ DEFAULT_CONFIG = {
         'disabled':     False,
         'filesystems':  ['tank'],
         'recursive':    True,
+        'if_modified':  False,
         'tag':          'zasd',
         'trigger':      IntervalTrigger(hours=12),
         'priority':     1,