rtc: Add useful timestamp definitions

Add commonly used timestamps for range definition.
Signed-off-by: NAlexandre Belloni <alexandre.belloni@bootlin.com>

include/linux/rtc.h

@@ -169,6 +169,11 @@ struct rtc_device {
 };
 #define to_rtc_device(d) container_of(d, struct rtc_device, dev)
 
+/* useful timestamps */
+#define RTC_TIMESTAMP_BEGIN_1900	-2208989361LL /* 1900-01-01 00:00:00 */
+#define RTC_TIMESTAMP_BEGIN_2000	946684800LL /* 2000-01-01 00:00:00 */
+#define RTC_TIMESTAMP_END_2099		4102444799LL /* 2099-12-31 23:59:59 */
+
 extern struct rtc_device *rtc_device_register(const char *name,
 					struct device *dev,
 					const struct rtc_class_ops *ops,