You can now find a previously scheduled time.

crontab/_crontab.py

@@ -50,6 +50,7 @@ WEEK = datetime.timedelta(days=7)
 MONTH = datetime.timedelta(days=28)
 YEAR = datetime.timedelta(days=365)
 
+# find the next scheduled time
 def _day_incr(dt, m):
     if m.day.input != 'l':
         return DAY
@@ -86,6 +87,40 @@ _increments = [
     _year_incr,
 ]
 
+# find the previously scheduled time
+def _day_decr(dt, m):
+    if m.day.input != 'l':
+        return -DAY
+    odt = dt
+    ndt = dt = dt - DAY
+    while dt.month == ndt.month:
+        dt -= DAY
+    return dt - odt
+
+def _month_decr(dt, m):
+    odt = dt
+    # get to the last day of last month, let the backtracking handle it
+    dt = dt.replace(day=1) - DAY
+    return dt - odt
+
+def _year_decr(dt, m):
+    # simple leapyear stuff works for 1970-2099 :)
+    mod = dt.year % 4
+    if mod == 0 and (dt.month, dt.day) > (2, 29):
+        return -(YEAR + DAY)
+    if mod == 1 and (dt.month, dt.day) < (2, 29):
+        return -(YEAR + DAY)
+    return -YEAR
+
+_decrements = [
+    lambda *a: -MINUTE,
+    lambda *a: -HOUR,
+    _day_decr,
+    _month_decr,
+    lambda *a: -DAY,
+    _year_decr,
+]
+
 Matcher = namedtuple('Matcher', 'minute, hour, day, month, weekday, year')
 
 def _assert(condition, message, *args):
@@ -112,6 +147,10 @@ class _Matcher(object):
         if self.any:
             return self.end < other
         return all(item < other for item in self.allowed)
+    def __gt__(self, other):
+        if self.any:
+            return _ranges[self.which][0] > other
+        return all(item > other for item in self.allowed)
     def _parse_crontab(self, which, entry):
         '''
         This parses a single crontab field and returns the data necessary for
@@ -220,7 +259,7 @@ class CronTab(object):
             attr = attr() % 7
         return self.matchers[index](attr, dt)
 
-    def next(self, now=None):
+    def next(self, now=None, increments=_increments):
         '''
         How long to wait in seconds before this crontab entry can next be
         executed.
@@ -228,14 +267,20 @@ class CronTab(object):
         now = now or datetime.datetime.now()
         if isinstance(now, (int, long, float)):
             now = datetime.datetime.fromtimestamp(now)
-        # get a reasonable future start time
-        future = now.replace(second=0, microsecond=0) + datetime.timedelta(minutes=1)
+        # get a reasonable future/past start time
+        future = now.replace(second=0, microsecond=0) + increments[0]()
+        if future < now:
+            # we are going backwards...
+            _test = lambda: future.year < self.matchers.year
+        else:
+            # we are going forwards
+            _test = lambda: self.matchers.year < future.year
         to_test = 0
         while to_test < 6:
-            incr = _increments[to_test]
+            incr = increments[to_test]
             while not self._test_match(to_test, future):
                 future += incr(future, self.matchers)
-                if self.matchers.year < future.year:
+                if _test():
                     return None
             # check for backtrack conditions
             if to_test >= 3:
@@ -255,3 +300,6 @@ class CronTab(object):
             "now: %r", ' '.join(m.input for m in self.matchers), now)
         delay = future - now
         return delay.days * 86400 + delay.seconds + delay.microseconds / 1000000.
+
+    def previous(self, now=None):
+        return self.next(now, _decrements)

setup.py

@@ -7,7 +7,7 @@ with open('README') as f:
 
 setup(
     name='crontab',
-    version='.11',
+    version='.12',
     description='Parse and use crontab schedules in Python',
     author='Josiah Carlson',
     author_email='josiah.carlson@gmail.com',

tests/test_crontab.py

@@ -11,6 +11,12 @@ class TestCrontab(unittest.TestCase):
         assert delay is not None
         dd = (crontab, delay, max_delay, now, now+datetime.timedelta(seconds=delay))
         assert delay <= max_delay, dd
+        if not crontab.endswith(' 2099'):
+            delay2 = ct.previous(now + datetime.timedelta(seconds=delay))
+            dd = (crontab, delay, max_delay, now, now+datetime.timedelta(seconds=delay))
+            assert abs(delay2) >= delay, (delay, delay2)
+            pt = now + datetime.timedelta(seconds=delay) + datetime.timedelta(seconds=delay2)
+            assert pt <= now, dd
 
     def _run_impossible(self, crontab, now):
         ct = CronTab(crontab)