Global site variables to be used in node fields and menu item I'm looking for some way of having variable defined by admin which can be displayed in some fields or nodes.

Real use case:
I have that specific date that changes periodically. And I want it to appear in some places, ie. within text of some node body (with possibility of fixing the variable ie. putting some text instead of date, but it's not a matter of my question). 

My guess: create some hook to preg_replace all occurences of some string, ie. ##VAR_VARNAME1## with certain fields of nodes or within field texts. But actually I know that it will occur mainly in 2 or 3 places within my website (which is for sure body of one specific node and text of 1 menu item).

Any ideas to make this efficient and quick?

edit: I believe it's easy to do with defining new block.. but how about way I mentioned above..
Try the Custom Tokens extension to the token module. Here is a quote about it (from the module's project page):

... gives the user the ability to create custom tokens for specific replacements that can improve other modules relying on the Drupal 7 token API or, for Drupal 6, the Token module.
Versions 7.x-1.x and previours relied on PHP evaluation to build custom tokens.
The new 7.x-2.x relies on input filters. While still allowing for PHP tokens through the D7 core PHP filter module, administrators can permit only HTML or plain text custom tokens.