diff --git a/rx/concurrency/schedulerbase.py b/rx/concurrency/schedulerbase.py
index c8d805690..af627695b 100644
--- a/rx/concurrency/schedulerbase.py
+++ b/rx/concurrency/schedulerbase.py
@@ -1,3 +1,4 @@
+from abc import abstractmethod
 from datetime import datetime, timedelta
 from typing import Optional
 
@@ -6,15 +7,94 @@
 from rx.internal.basic import default_now
 from rx.internal.constants import DELTA_ZERO, UTC_ZERO
 
+
 class SchedulerBase(typing.Scheduler):
     """Provides a set of static properties to access commonly used
     schedulers.
     """
 
+    @property
+    def now(self) -> datetime:
+        """Represents a notion of time for this scheduler. Tasks being
+        scheduled on a scheduler will adhere to the time denoted by this
+        property.
+
+        Returns:
+             The scheduler's current time, as a datetime instance.
+         """
+
+        return default_now()
+
+    @abstractmethod
+    def schedule(self,
+                 action: typing.ScheduledAction,
+                 state: Optional[typing.TState] = None
+                 ) -> typing.Disposable:
+        """Schedules an action to be executed.
+
+        Args:
+            action: Action to be executed.
+            state: [Optional] state to be given to the action function.
+
+        Returns:
+            The disposable object used to cancel the scheduled action
+            (best effort).
+        """
+        return NotImplemented
+
+    @abstractmethod
+    def schedule_relative(self,
+                          duetime: typing.RelativeTime,
+                          action: typing.ScheduledAction,
+                          state: Optional[typing.TState] = None
+                          ) -> typing.Disposable:
+        """Schedules an action to be executed after duetime.
+
+        Args:
+            duetime: Relative time after which to execute the action.
+            action: Action to be executed.
+            state: [Optional] state to be given to the action function.
+
+        Returns:
+            The disposable object used to cancel the scheduled action
+            (best effort).
+        """
+        return NotImplemented
+
+    @abstractmethod
+    def schedule_absolute(self,
+                          duetime: typing.AbsoluteTime,
+                          action: typing.ScheduledAction,
+                          state: Optional[typing.TState] = None
+                          ) -> typing.Disposable:
+        """Schedules an action to be executed at duetime.
+
+        Args:
+            duetime: Absolute time after which to execute the action.
+            action: Action to be executed.
+            state: [Optional] state to be given to the action function.
+
+        Returns:
+            The disposable object used to cancel the scheduled action
+            (best effort).
+        """
+        return NotImplemented
+
     def invoke_action(self,
                       action: typing.ScheduledAction,
                       state: Optional[typing.TState] = None
                       ) -> typing.Disposable:
+        """Invoke the given given action. This is typically called by instances
+        of ScheduledItem.
+
+        Args:
+            action: Action to be executed.
+            state: [Optional] state to be given to the action function.
+
+        Returns:
+            The disposable object returned by the action, if any; or a new
+            (no-op) disposable otherwise.
+        """
         ret = action(self, state)
         if isinstance(ret, typing.Disposable):
             return ret
@@ -65,69 +145,89 @@ def invoke_periodic(scheduler: typing.Scheduler, _: typing.TState) -> Optional[D
         disp.disposable = self.schedule_relative(period, invoke_periodic, None)
         return disp
 
-    @property
-    def now(self) -> datetime:
-        """Returns the current time.
+    @classmethod
+    def to_seconds(cls, value: typing.AbsoluteOrRelativeTime) -> float:
+        """Converts time value to seconds. This method handles both absolute
+        (datetime) and relative (timedelta) values. If the argument is already
+        a float, it is simply returned unchanged.
 
-        Represents a notion of time for this scheduler. Tasks being
-        scheduled on a scheduler will adhere to the time denoted by
-        this property.
-        """
+        Args:
+            value: the time value to convert to seconds.
 
-        return default_now()
+        Returns:
+            The value converted to seconds.
+        """
 
-    @classmethod
-    def to_seconds(cls, timespan: typing.AbsoluteOrRelativeTime) -> float:
-        """Converts time value to seconds"""
+        if isinstance(value, datetime):
+            value = value - UTC_ZERO
 
-        if isinstance(timespan, datetime):
-            timespan = timespan - UTC_ZERO
-            timespan = timespan.total_seconds()
-        elif isinstance(timespan, timedelta):
-            timespan = timespan.total_seconds()
+        if isinstance(value, timedelta):
+            value = value.total_seconds()
 
-        return timespan
+        return value
 
     @classmethod
-    def to_datetime(cls, duetime: typing.AbsoluteOrRelativeTime) -> datetime:
-        """Converts time value to datetime"""
+    def to_datetime(cls, value: typing.AbsoluteOrRelativeTime) -> datetime:
+        """Converts time value to datetime. This method handles both absolute
+        (float) and relative (timedelta) values. If the argument is already
+        a datetime, it is simply returned unchanged.
+
+        Args:
+            value: the time value to convert to datetime.
+
+        Returns:
+            The value converted to datetime.
+        """
 
-        if isinstance(duetime, timedelta):
-            duetime = UTC_ZERO + duetime
-        elif not isinstance(duetime, datetime):
-            duetime = datetime.utcfromtimestamp(duetime)
+        if isinstance(value, timedelta):
+            value = UTC_ZERO + value
+        elif not isinstance(value, datetime):
+            value = datetime.utcfromtimestamp(value)
 
-        return duetime
+        return value
 
     @classmethod
-    def to_timedelta(cls, timespan: typing.AbsoluteOrRelativeTime) -> timedelta:
-        """Converts time value to timedelta"""
+    def to_timedelta(cls, value: typing.AbsoluteOrRelativeTime) -> timedelta:
+        """Converts time value to timedelta. This method handles both absolute
+        (datetime) and relative (float) values. If the argument is already
+        a timedelta, it is simply returned unchanged. If the argument is an
+        absolute time, the result value will be the timedelta since the epoch,
+        January 1st, 1970, 00:00:00.
+
+        Args:
+            value: the time value to convert to timedelta.
+
+        Returns:
+            The value converted to timedelta.
+        """
 
-        if isinstance(timespan, datetime):
-            timespan = timespan - UTC_ZERO
-        elif not isinstance(timespan, timedelta):
-            timespan = timedelta(seconds=timespan)
+        if isinstance(value, datetime):
+            value = value - UTC_ZERO
+        elif not isinstance(value, timedelta):
+            value = timedelta(seconds=value)
 
-        return timespan
+        return value
 
     @classmethod
-    def normalize(cls, timespan: typing.RelativeTime) -> typing.RelativeTime:
-        """Normalizes the specified timespan value to a positive value.
+    def normalize(cls, value: typing.RelativeTime) -> typing.RelativeTime:
+        """Normalizes the specified time value to a non-negative value. This
+        method handles only relative values, given as either timedelta or float,
+        and will return the normalized value as that same type.
 
         Args:
-            timespan: The time span value to normalize.
+            value: The time value to normalize.
 
         Returns:
             The specified timespan value if it is zero or positive;
             otherwise, 0.0
         """
 
-        if isinstance(timespan, timedelta):
-            if not timespan or timespan < DELTA_ZERO:
+        if isinstance(value, timedelta):
+            if not value or value < DELTA_ZERO:
                 return DELTA_ZERO
 
-        elif isinstance(timespan, float):
-            if not timespan or timespan < 0.0:
+        elif isinstance(value, float):
+            if not value or value < 0.0:
                 return 0.0
 
-        return timespan
+        return value
diff --git a/rx/core/abc/scheduler.py b/rx/core/abc/scheduler.py
index 109af102f..28eca0aae 100644
--- a/rx/core/abc/scheduler.py
+++ b/rx/core/abc/scheduler.py
@@ -20,3 +20,6 @@ def schedule_relative(self, duetime, action, state=None):
     @abstractmethod
     def schedule_absolute(self, duetime, action, state=None):
         return NotImplemented
+
+    def schedule_periodic(self, period, action, state=None):
+        return NotImplemented
diff --git a/rx/core/typing.py b/rx/core/typing.py
index d5fe622f8..c6b3a1b33 100644
--- a/rx/core/typing.py
+++ b/rx/core/typing.py
@@ -47,15 +47,34 @@ def now(self) -> datetime:
         return NotImplemented
 
     @abstractmethod
-    def schedule(self, action: 'ScheduledAction', state: TState = None) -> Disposable:
+    def schedule(self,
+                 action: 'ScheduledAction',
+                 state: Optional[TState] = None
+                 ) -> Disposable:
         return NotImplemented
 
     @abstractmethod
-    def schedule_relative(self, duetime: RelativeTime, action: 'ScheduledAction', state: TState = None) -> Disposable:
+    def schedule_relative(self,
+                          duetime: RelativeTime,
+                          action: 'ScheduledAction',
+                          state: Optional[TState] = None
+                          ) -> Disposable:
         return NotImplemented
 
     @abstractmethod
-    def schedule_absolute(self, duetime: AbsoluteTime, action: 'ScheduledAction', state: TState = None) -> Disposable:
+    def schedule_absolute(self,
+                          duetime: AbsoluteTime,
+                          action: 'ScheduledAction',
+                          state: Optional[TState] = None
+                          ) -> Disposable:
+        return NotImplemented
+
+    @abstractmethod
+    def schedule_periodic(self,
+                          period: RelativeTime,
+                          action: 'ScheduledPeriodicAction',
+                          state: Optional[TState] = None
+                          ) -> Disposable:
         return NotImplemented
 
 
diff --git a/tests/test_concurrency/test_schedulerbase.py b/tests/test_concurrency/test_schedulerbase.py
new file mode 100644
index 000000000..7e6625c32
--- /dev/null
+++ b/tests/test_concurrency/test_schedulerbase.py
@@ -0,0 +1,51 @@
+import unittest
+from datetime import timedelta
+
+from rx.internal.constants import DELTA_ZERO, UTC_ZERO
+from rx.concurrency.schedulerbase import SchedulerBase
+
+
+class TestSchedulerBase(unittest.TestCase):
+
+    def test_base_to_seconds(self):
+        val = SchedulerBase.to_seconds(0.0)
+        assert val == 0.0
+        val = SchedulerBase.to_seconds(DELTA_ZERO)
+        assert val == 0.0
+        val = SchedulerBase.to_seconds(UTC_ZERO)
+        assert val == 0.0
+
+    def test_base_to_datetime(self):
+        val = SchedulerBase.to_datetime(0.0)
+        assert val == UTC_ZERO
+        val = SchedulerBase.to_datetime(DELTA_ZERO)
+        assert val == UTC_ZERO
+        val = SchedulerBase.to_datetime(UTC_ZERO)
+        assert val == UTC_ZERO
+
+    def test_base_to_timedelta(self):
+        val = SchedulerBase.to_timedelta(0.0)
+        assert val == DELTA_ZERO
+        val = SchedulerBase.to_timedelta(DELTA_ZERO)
+        assert val == DELTA_ZERO
+        val = SchedulerBase.to_timedelta(UTC_ZERO)
+        assert val == DELTA_ZERO
+
+    def test_base_normalize_float(self):
+        val = SchedulerBase.normalize(-1.0)
+        assert val == 0.0
+        val = SchedulerBase.normalize(0.0)
+        assert val == 0.0
+        val = SchedulerBase.normalize(1.0)
+        assert val == 1.0
+
+    def test_base_normalize_delta(self):
+        DELTA_ONE = timedelta(seconds=1.0)
+        val = SchedulerBase.normalize(-DELTA_ONE)
+        assert val == DELTA_ZERO
+        val = SchedulerBase.normalize(DELTA_ZERO)
+        assert val == DELTA_ZERO
+        val = SchedulerBase.normalize(DELTA_ONE)
+        assert val == DELTA_ONE
+
+