diff --git a/download.py b/download.py
index e907085..6d57353 100644
--- a/download.py
+++ b/download.py
@@ -1,6 +1,9 @@
-#!/usr/bin/python3
-import shutil
+# !/usr/bin/python3
+import argparse
+from datetime import date
+from datetime import datetime, timedelta
 from os.path import join, exists
+import shutil
 from urllib.request import urlopen
 
 from skyfield_data import get_skyfield_data_path
@@ -9,12 +12,83 @@ JPL = "ftp://ssd.jpl.nasa.gov/pub/eph/planets/bsp"
 USNO = "http://maia.usno.navy.mil/ser7"
 IERS = "https://hpiers.obspm.fr/iers/bul/bulc"
 
-items = (
-    ("de421.bsp", JPL),
-    ("deltat.data", USNO),
-    ("deltat.preds", USNO),
-    ("Leap_Second.dat", IERS),
-)
+
+def deltat_data_expiration(fileobj):
+    """Return the expiration date for the USNO ``deltat.data`` file.
+
+    Each line file gives the date and the value of Delta T::
+
+        2016  2  1  68.1577
+    """
+    line = None
+    for line in fileobj.readlines():
+        pass
+    # `line` is the last line
+    if not line:  # No line, error...
+        return
+    array = line.strip().split()
+    # We'll only keep the year / month / day
+    array = array[:3]
+    array = map(int, array)
+    year, month, day = array
+    # By convention, the file expires at year+1
+    return date(year + 1, month, day)
+
+
+def deltat_preds_expiration(fileobj):
+    """
+    Return the expiration date for the USNO ``deltat.preds`` file.
+
+    The old format supplies a floating point year, the value of Delta T,
+    and one or two other fields::
+
+    2015.75      67.97               0.210         0.02
+
+    The new format adds a modified Julian day as the first field:
+
+    58484.000  2019.00   69.34      -0.152       0.117
+
+    """
+    lines = iter(fileobj)
+    header = next(lines)
+
+    if header.startswith(b'YEAR'):
+        # Format in use until 2019 February
+        next(lines)  # discard blank line
+        line = next(lines)
+        year_float = float(line.strip().split().pop())
+    else:
+        # Format in use since 2019 February
+        line = next(lines)
+        array = line.strip().split()
+        year_float = float(array[1])
+
+    year = int(year_float)
+    month = 1 + int(year_float * 12.0) % 12
+    expiration_date = date(year + 2, month, 1)
+    return expiration_date
+
+
+def leap_seconds_expiration(fileobj):
+    """
+    Return the expiration date for the IERS file ``Leap_Second.dat``.
+    """
+    lines = iter(fileobj)
+    for line in lines:
+        if line.startswith(b'#  File expires on'):
+            break
+    else:
+        raise ValueError('Leap_Second.dat is missing its expiration date')
+    line = line.decode('ascii')
+
+    dt = datetime.strptime(line, '#  File expires on %d %B %Y\n')
+
+    # The file went out of date at the beginning of July 2016, and kept
+    # downloading every time a user ran a Skyfield program.  So we now
+    # build in a grace period:
+    grace_period = timedelta(days=30)
+    expiration_date = dt.date() + grace_period
+    return expiration_date
 
 
 def download(url, target):
@@ -25,15 +99,75 @@ def download(url, target):
             shutil.copyfileobj(response, fd)
 
 
-if __name__ == '__main__':
-    for filename, server in items:
+def check_should_i_download(target, params):
+    """
+    Check if I should download the target file or not.
+    """
+    should_i_download = True
+    reason = None
+
+    if exists(target):
+        # Search if the file has expired
+        expiration_func = params.get("expiration_func")
+        if expiration_func:
+            with open(target, 'rb') as fd:
+                expiration_date = expiration_func(fd)
+            if date.today() <= expiration_date:
+                should_i_download = False
+                reason = 'file will expire at {}'.format(expiration_date)
+            else:
+                reason = "expiration date: {}".format(expiration_date)
+        else:
+            should_i_download = False
+            reason = "file already exists, no expiration function"
+    else:
+        reason = "file not here"
+
+    return should_i_download, reason
+
+
+def main(args):
+    items = {
+        "de421.bsp": {"server": JPL},
+        "deltat.data": {
+            "server": USNO,
+            "expiration_func": deltat_data_expiration
+        },
+        "deltat.preds": {
+            "server": USNO,
+            "expiration_func": deltat_preds_expiration
+        },
+        "Leap_Second.dat": {
+            "server": IERS,
+            "expiration_func": leap_seconds_expiration
+        },
+    }
+
+    for filename, params in items.items():
+        server = params["server"]
         url = "{}/{}".format(server, filename)
         target = join(get_skyfield_data_path(), filename)
 
-        if not exists(target):
-            print("Downloading {}".format(filename))
+        if args.force:
+            should_i_download, reason = True, "Forced download"
+        else:
+            should_i_download, reason = check_should_i_download(
+                target, params
+            )
+
+        if should_i_download:
+            print("Downloading {} ({})".format(filename, reason))
             download(url, target)
         else:
-            print("Skipping {}".format(filename))
+            print("Skipping {} ({})".format(filename, reason))
+    print("\nDone\n")
 
-    print("OK")
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser("Download Skyfield files")
+    parser.add_argument(
+        '--force', action="store_true", default=False,
+        help="Force download, ignore expiration date or presence on disk."
+    )
+    args = parser.parse_args()
+    main(args)