diff --git a/Makefile b/Makefile
index 691d0bd..2d4aee8 100644
--- a/Makefile
+++ b/Makefile
@@ -15,7 +15,7 @@ CPPFLAGS += $(foreach dir, $(SUBDIRS), -I$(dir))
 asmsrc=$(wildcard *.asm)
 asmobj=$(asmsrc:%.asm=%.o)
 csrc=$(shell find $(SUBDIRS) -type f -name "*.c")#   $(wildcard *.c)
-cobj=$(csrc:%.c=%.o) core/cpu_context_switch.o core/irq_pit.o
+cobj=$(csrc:%.c=%.o) core/cpu_context_switch.o drivers/irq_pit.o
 deps = $(csrc:%.c=%.d)
 
 kernel:$(asmobj) $(cobj) linker.ld
diff --git a/core/kthread.c b/core/kthread.c
index 339d794..768378d 100644
--- a/core/kthread.c
+++ b/core/kthread.c
@@ -4,6 +4,7 @@
 #include "irq.h"
 #include "klibc.h"
 #include "list.h"
+#include "time.h"
 #include "vga.h"
 
 static struct kthread *currentThread;
@@ -80,7 +81,7 @@ struct kthread *kthreadSelectNext()
     return nextThread;
 }
 
-struct cpu_state *switchKthread(struct cpu_state *prevCpu)
+struct cpu_state *kthreadSwitch(struct cpu_state *prevCpu)
 {
     uint32_t flags;
     disable_IRQs(flags);
diff --git a/core/kthread.h b/core/kthread.h
index d2279f9..2317af4 100644
--- a/core/kthread.h
+++ b/core/kthread.h
@@ -22,6 +22,6 @@ struct kthread *kthreadCreate(const char *name, cpu_kstate_function_arg1_t func,
 void kthreadDelete(struct kthread *thread);
 
 struct kthread *kthreadSelectNext();
-struct cpu_state *switchKthread(struct cpu_state *prevCpu);
+struct cpu_state *kthreadSwitch(struct cpu_state *prevCpu);
 
 int kthreadYield();
diff --git a/core/main.c b/core/main.c
index 03c8ce4..d29fbe2 100644
--- a/core/main.c
+++ b/core/main.c
@@ -17,6 +17,7 @@
 #ifdef RUN_TEST
 #include "test.h"
 #endif
+#include "time.h"
 #include "types.h"
 #include "vga.h"
 
@@ -29,9 +30,8 @@ void cpuid(int code, uint32_t *a, uint32_t *d)
 void idleThread(void *arg)
 {
     (void)arg;
-    int count = 0;
     while (1)
-        printIntDetails(count++, GREEN, BLACK, 0, VGA_HEIGHT - 1);
+        printIntDetails((jiffies/HZ), GREEN, BLACK, 0, VGA_HEIGHT - 1);
 }
 
 // Multiboot information available here :
@@ -47,7 +47,7 @@ void kmain(unsigned long magic, unsigned long addr)
     gdtSetup();
     idtSetup();
     irqSetup();
-    pitSetup(100);
+    pitSetup(HZ);
 
     if (magic == MULTIBOOT_BOOTLOADER_MAGIC) { // Get loaded by Grub with mutliboot version 1
         multiboot_info_t *mbi = (multiboot_info_t *)addr;
diff --git a/core/time.c b/core/time.c
new file mode 100644
index 0000000..cb0c607
--- /dev/null
+++ b/core/time.c
@@ -0,0 +1,13 @@
+#include "time.h"
+
+unsigned long volatile jiffies = INITIAL_JIFFIES;
+
+unsigned int jiffies_to_msecs(const unsigned long j)
+{
+    return (1000L / HZ) * j;
+}
+
+unsigned int jiffies_to_usecs(const unsigned long j)
+{
+    return (1000000L / HZ) * j;
+}
diff --git a/core/time.h b/core/time.h
new file mode 100644
index 0000000..302aa94
--- /dev/null
+++ b/core/time.h
@@ -0,0 +1,50 @@
+#pragma once
+
+#define HZ 100
+/*
+ * Have the 32 bit jiffies value wrap 5 minutes after boot
+ * so jiffies wrap bugs show up earlier.
+ */
+//#define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))
+#define INITIAL_JIFFIES 0
+
+extern unsigned long volatile jiffies;
+
+
+#define time_after(unknown, known) ((long)(known) - (long)(unknown) < 0)
+#define time_before(unknown, known) ((long)(unknown) - (long)(known) < 0)
+#define time_after_eq(unknown, known) ((long)(unknown) - (long)(known) >= 0)
+#define time_before_eq(unknown, known) ((long)(known) - (long)(unknown) >= 0)
+
+/*
+ * Calculate whether a is in the range of [b, c].
+ */
+#define time_in_range(a,b,c) \
+	(time_after_eq(a,b) && \
+	 time_before_eq(a,c))
+
+/*
+ * Calculate whether a is in the range of [b, c).
+ */
+#define time_in_range_open(a,b,c) \
+	(time_after_eq(a,b) && \
+	 time_before(a,c))
+
+/*
+ * These four macros compare jiffies and 'a' for convenience.
+ */
+
+/* time_is_before_jiffies(a) return true if a is before jiffies */
+#define time_is_before_jiffies(a) time_after(jiffies, a)
+
+/* time_is_after_jiffies(a) return true if a is after jiffies */
+#define time_is_after_jiffies(a) time_before(jiffies, a)
+
+/* time_is_before_eq_jiffies(a) return true if a is before or equal to jiffies*/
+#define time_is_before_eq_jiffies(a) time_after_eq(jiffies, a)
+
+/* time_is_after_eq_jiffies(a) return true if a is after or equal to jiffies*/
+#define time_is_after_eq_jiffies(a) time_before_eq(jiffies, a)
+
+unsigned int jiffies_to_msecs(const unsigned long j);
+unsigned int jiffies_to_usecs(const unsigned long j);
diff --git a/core/irq_pit.S b/drivers/irq_pit.S
similarity index 97%
rename from core/irq_pit.S
rename to drivers/irq_pit.S
index ac7b935..039cb7a 100644
--- a/core/irq_pit.S
+++ b/drivers/irq_pit.S
@@ -21,7 +21,7 @@ pit_handler:         // already got eflags, cs and eip on stack thanks to CPU
   outb  %al, $0x20
 
   pushl %esp
-  call switchKthread
+  call pitIrqHandler
   movl %eax,%esp
 
   /* Restore the CPU context */
diff --git a/drivers/pit.c b/drivers/pit.c
index f0a3fdf..b79469a 100644
--- a/drivers/pit.c
+++ b/drivers/pit.c
@@ -1,6 +1,9 @@
 #include "pit.h"
 #include "io.h"
 #include "irq.h"
+#include "kthread.h"
+#include "time.h"
+#include "klibc.h"
 
 int pitSetup(unsigned int freq)
 {
@@ -13,3 +16,9 @@ int pitSetup(unsigned int freq)
 
     return 0;
 }
+
+struct cpu_state *pitIrqHandler(struct cpu_state *prevCpu)
+{
+    __atomic_add_fetch(&jiffies, 1, __ATOMIC_RELAXED);
+    return kthreadSwitch(prevCpu);
+}
diff --git a/drivers/pit.h b/drivers/pit.h
index c72a257..3fc9ce3 100644
--- a/drivers/pit.h
+++ b/drivers/pit.h
@@ -1,4 +1,5 @@
 #pragma once
+#include "cpu_context.h"
 
 // C.f https://wiki.osdev.org/PIT
 
@@ -13,3 +14,4 @@
 // 3 -> low then high 3-1: mode. See https://wiki.osdev.org/PIT
 
 int pitSetup(unsigned int freq);
+struct cpu_state *pitIrqHandler(struct cpu_state *prevCpu);