add note about the effects of time changes (bug #4020)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/v1-0@5617 65c4cc65-6c06-0410-ace0-fbb531ad65f3

diff --git a/README b/README
index 64bf06a2096da8d9ec7883984f8011593ce50d8d..3a78bc9bea5dc8b8290e61bc9f625d14c81da705 100755 (executable)
--- a/README
+++ b/README
@@ -136,6 +136,30 @@ parameters.  For example, the line "channel => 25-47" creates objects for
 the channels 25 through 47 of the tormenta card, obtaining the settings
 from the variables specified above.
 
+* SPECIAL NOTE ON TIME
+  
+  Those using SIP phones should be aware the Asterisk is sensitive to
+large jumps in time.  Manually changing the system time using date(1)
+(or other similar commands) may cause SIP registrations and other
+internal processes to fail.  If your system cannot keep accurate time
+by itself use NTP (http://www.ntp.org/) to keep the system clock
+synchronized to "real time".  NTP is designed to keep the system clock
+synchronized by speeding up or slowing down the system clock until it
+is synchronized to "real time" rather than by jumping the time and
+causing discontinuities. Most Linux distributions include precompiled
+versions of NTP.  Beware of some time synchronization methods that get
+the correct real time periodically and then manually set the system
+clock.
+
+  Apparent time changes due to daylight savings time are just that,
+apparent.  The use of daylight savings time in a Linux system is
+purely a user interface issue and does not affect the operation of the
+Linux kernel or Asterisk.  The system clock on Linux kernels operates
+on UTC.  UTC does not use daylight savings time.
+   
+  Also note that this issue is separate from the clocking of TDM
+channels, and is known to at least affect SIP registrations.
+
 * MORE INFORMATION
 
 See the doc directory for more documentation.