diff --git a/artiq/language/experiment.py b/artiq/language/experiment.py
index 76a51e1f1..ef2f80269 100644
--- a/artiq/language/experiment.py
+++ b/artiq/language/experiment.py
@@ -44,6 +44,8 @@ class _DummyTimeManager:
 	enter_sequential = _not_implemented
 	exit = _not_implemented
 	take_time = _not_implemented
+	get_time = _not_implemented
+	set_time = _not_implemented
 
 _time_manager = _DummyTimeManager()
 
@@ -70,4 +72,10 @@ class _Parallel:
 parallel = _Parallel()
 
 def delay(duration):
-	_time_manager.take_time(duration.amount)
+	_time_manager.take_time(duration)
+
+def now():
+	return _time_manager.get_time()
+
+def at(time):
+	_time_manager.set_time(time)
diff --git a/artiq/sim/devices.py b/artiq/sim/devices.py
index 2e28149ba..bc6621c05 100644
--- a/artiq/sim/devices.py
+++ b/artiq/sim/devices.py
@@ -19,13 +19,12 @@ class Input:
 	def wait_edge(self):
 		duration = self.prng.randrange(self.wait_min, self.wait_max)*units.ms
 		time.manager.event(("wait_edge", self.name, duration))
-		time.manager.take_time(duration.amount)
+		time.manager.take_time(duration)
 
 	def count_gate(self, duration):
 		result = self.prng.randrange(self.count_min, self.count_max)
-		units.check_unit(duration, units.base_s_unit)
 		time.manager.event(("count_gate", self.name, duration, result))
-		time.manager.take_time(duration.amount)
+		time.manager.take_time(duration)
 		return result
 
 class WaveOutput:
@@ -33,10 +32,8 @@ class WaveOutput:
 		self.name = name
 
 	def pulse(self, frequency, duration):
-		units.check_unit(frequency, units.base_Hz_unit)
-		units.check_unit(duration, units.base_s_unit)
 		time.manager.event(("pulse", self.name, frequency, duration))
-		time.manager.take_time(duration.amount)
+		time.manager.take_time(duration)
 
 class VoltageOutput:
 	def __init__(self, name):
diff --git a/artiq/sim/time.py b/artiq/sim/time.py
index 87b84e96f..dfcb4cdc8 100644
--- a/artiq/sim/time.py
+++ b/artiq/sim/time.py
@@ -1,11 +1,12 @@
 from operator import itemgetter
 
-from artiq.language import units, experiment
+from artiq.language.units import *
+from artiq.language import experiment
 
 class SequentialTimeContext:
 	def __init__(self, current_time):
 		self.current_time = current_time
-		self.block_duration = 0
+		self.block_duration = 0*ps
 
 	def take_time(self, amount):
 		self.current_time += amount
@@ -14,23 +15,23 @@ class SequentialTimeContext:
 class ParallelTimeContext:
 	def __init__(self, current_time):
 		self.current_time = current_time
-		self.block_duration = 0
+		self.block_duration = 0*ps
 
 	def take_time(self, amount):
 		if amount > self.block_duration:
-			self.block_duration = amount	
+			self.block_duration = amount
 
 class Manager:
 	def __init__(self):
-		self.stack = [SequentialTimeContext(0)]
+		self.stack = [SequentialTimeContext(0*ps)]
 		self.timeline = []
 
 	def enter_sequential(self):
-		new_context = SequentialTimeContext(self.stack[-1].current_time)
+		new_context = SequentialTimeContext(self.get_time())
 		self.stack.append(new_context)
 
 	def enter_parallel(self):
-		new_context = ParallelTimeContext(self.stack[-1].current_time)
+		new_context = ParallelTimeContext(self.get_time())
 		self.stack.append(new_context)
 
 	def exit(self):
@@ -40,16 +41,23 @@ class Manager:
 	def take_time(self, duration):
 		self.stack[-1].take_time(duration)
 
+	def get_time(self):
+		return self.stack[-1].current_time
+
+	def set_time(self, t):
+		dt = t - self.get_time()
+		if dt < 0*ps:
+			raise ValueError("Attempted to go back in time")
+		self.take_time(dt)
+
 	def event(self, description):
-		self.timeline.append((self.stack[-1].current_time, description))
+		self.timeline.append((self.get_time(), description))
 
 	def format_timeline(self):
 		r = ""
-		prev_time = 0
+		prev_time = 0*ps
 		for time, description in sorted(self.timeline, key=itemgetter(0)):
-			t = units.Quantity(time, units.base_s_unit)
-			dt = units.Quantity(time-prev_time, units.base_s_unit)
-			r += "@{:10} (+{:10}) ".format(str(t), str(dt))
+			r += "@{:10} (+{:10}) ".format(str(time), str(time-prev_time))
 			for item in description:
 				r += "{:16}".format(str(item))
 			r += "\n"