added the prevent_drift parameter

gluon/scheduler.py

@@ -507,6 +507,8 @@ class Scheduler(MetaScheduler):
                   requires=IS_INT_IN_RANGE(-1, None)),
             Field('period', 'integer', default=60, comment='seconds',
                   requires=IS_INT_IN_RANGE(0, None)),
+            Field('prevent_drift', 'boolean', default=False,
+                  comment='Cron-like start_times between runs'),
             Field('timeout', 'integer', default=60, comment='seconds',
                   requires=IS_INT_IN_RANGE(0, None)),
             Field('sync_output', 'integer', default=0,
@@ -635,9 +637,15 @@ class Scheduler(MetaScheduler):
             else:
                 logger.info('nothing to do')
             return None
-        next_run_time = task.last_run_time + datetime.timedelta(
-            seconds=task.period)
         times_run = task.times_run + 1
+        if not task.prevent_drift:
+            next_run_time = task.last_run_time + datetime.timedelta(
+                seconds=task.period
+                )
+        else:
+            next_run_time = task.start_time + datetime.timedelta(
+                seconds=task.period * times_run
+            )
         if times_run < task.repeats or task.repeats == 0:
             #need to run (repeating task)
             run_again = True