diff --git a/artiq/compiler/lower_time.py b/artiq/compiler/lower_time.py
new file mode 100644
index 000000000..00039dfad
--- /dev/null
+++ b/artiq/compiler/lower_time.py
@@ -0,0 +1,42 @@
+import ast
+
+class _TimeLowerer(ast.NodeTransformer):
+	def __init__(self, ref_period):
+		self.ref_period = ref_period
+
+	def visit_Call(self, node):
+		if isinstance(node.func, ast.Name) \
+		  and node.func.id == "Quantity" and node.args[1].id == "base_s_unit":
+			return ast.copy_location(
+				ast.BinOp(left=node.args[0], op=ast.FloorDiv(), right=ast.Num(self.ref_period.amount)),
+				node)
+		elif isinstance(node.func, ast.Name) and node.func.id == "now":
+			return ast.copy_location(ast.Name("now", ast.Load()), node)
+		else:
+			self.generic_visit(node)
+			return node
+
+	def visit_Expr(self, node):
+		self.generic_visit(node)
+		if isinstance(node.value, ast.Call) and isinstance(node.value.func, ast.Name):
+			funcname = node.value.func.id
+			if funcname == "delay":
+				return ast.copy_location(
+					ast.AugAssign(target=ast.Name("now", ast.Store()), op=ast.Add(), value=node.value.args[0]),
+					node)
+			elif funcname == "at":
+				return ast.copy_location(
+					ast.Assign(targets=[ast.Name("now", ast.Store())], value=node.value.args[0]),
+					node)
+			else:
+				return node
+		else:
+			return node
+
+def lower_time(stmts, ref_period):
+	transformer = _TimeLowerer(ref_period)
+	new_stmts = [transformer.visit(stmt) for stmt in stmts]
+	new_stmts.insert(0, ast.copy_location(
+		ast.Assign(targets=[ast.Name("now", ast.Store())], value=ast.Num(0)),
+		stmts[0]))
+	stmts[:] = new_stmts
diff --git a/artiq/devices/core.py b/artiq/devices/core.py
index 5bfa7bfbc..bc6eb4dff 100644
--- a/artiq/devices/core.py
+++ b/artiq/devices/core.py
@@ -2,6 +2,7 @@ from artiq.compiler.inline import inline
 from artiq.compiler.fold_constants import fold_constants
 from artiq.compiler.unroll_loops import unroll_loops
 from artiq.compiler.interleave import interleave
+from artiq.compiler.lower_time import lower_time
 from artiq.compiler.ir import get_runtime_binary
 
 class Core:
@@ -16,6 +17,8 @@ class Core:
 		fold_constants(stmts)
 		unroll_loops(stmts, 50)
 		interleave(stmts)
+		lower_time(stmts, self.runtime_env.ref_period)
+		fold_constants(stmts)
 
 		binary = get_runtime_binary(self.runtime_env, stmts)
 		self.core_com.run(binary)