Default Timezone setting in WordPress - is that global?

Question

Despite the WordPress Options' Timezone setting for the whole WordPress installation, is it necessary to declare the default timezone in all the date/time places in a WP theme/plugins?

date_default_timezone_set('Asia/Dhaka');
echo date('d M Y - D - h:i A');

Suppose I'm handling with something where I'm checking whether the date&time is equals to TODAY. You know what "Today" means is a bit variable - variable to server time, variable to local time etc.

That's why I'm looking for a standard procedure to follow in WP projects, whether to set the default timezone each and every time (or do I call them always in the header?) or to avoid them as WordPress itself declared that already (just tune that before).

I'm just looking for a standard procedure - what to follow...