Manual:$wgCacheEpoch

Cache: $wgCacheEpoch
Set this to current time to invalidate all prior cached pages.
Introduced in version:	pre 1.1.0
Removed in version:	still in use
Allowed values:	Date/Time string
Default value:	'20030516000000'

Other settings: Alphabetical | By Function

Details

Set this to current time to invalidate all prior cached pages. Affects both client- and server-side caching, including assets controlled by ResourceLoader.

For a quick copy-paste you can use {{CURRENTTIMESTAMP}}: 20141125163332

If you are using Linux you can get the current date by entering the following on the command-line:

date +%Y%m%d%H%M%S

To force a file, such as LocalSettings.php to be re-read when touched, you can use this code (taken from LocalSettings.php, MW < 1.17).

# When you make changes to this configuration file, this will make                                       
# sure that cached pages are cleared.                                                                    
$wgCacheEpoch = max( $wgCacheEpoch, gmdate( 'YmdHis', @filemtime( __FILE__ ) ) );

Since version 1.17, you can use $wgInvalidateCacheOnLocalSettingsChange to automatically invalidate cache whenever LocalSettings.php is modified. In earlier versions, code in LocalSettings.php did a similar thing.