My web host recently advised me that my site was being migrated (after the event). I am led to believe that this is in the interests of improving the service.
Immediately after the migration I discovered that one php file using date/time functions was stalling and numerous files were showing an errorWarning: phpinfo() [function.phpinfo]: It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'Antarctica/Macquarie' for 'EST/10.0/no DST' instead in /home/safetyte/public_html/php_info.php on line 1.
After prolonged discussions, support advised me to add date_default_timezone_set('Australia/Melbourne'); immediately after session_start();. This did resolve the problem (that did not exist previously). As well as that, php.ini shows date.timezone as having no value. I raised this issue with support and was instructed to add the following to a htaccess file which I was to set up in the root.
Everything now works OK again, but I have a lingering feeling that I may have just been "sold a pup"
Can anyone who understands these things make comment?
I wonder if I've been moved to some sort of shared server.

All these things you mention are the result of an upgrade of PHP. My guess is they upgraded to 5.4, which triggers these messages.

I have the same version PHP 5.4.3 on my WAMP server and there are none of these messages.

Quite possibly because WAMP server comes preinstalled with a matching PHP.INI. Upgrading a PHP version does not automatically add these required changes.

You can make the change in the php.ini file, but you can alternatively make the change at the top of the PHP script with something like this: date_default_timezone_set('America/New_York');