diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6fd31b832..58162daed 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,4 +18,7 @@ Master branch
 
 - fix error when interval is hour only
 - layout of page "list exports"
+- create function "SIMILARITY" (see [posgtgresql documentation](https://www.postgresql.org/docs/9.6/static/pgtrgm.html)) 
+- create function "OVERLAPSI", which will detect period of date overlapping, replacing NULL date by infinity or -infinity (see [postgresql page for date time function and operators](https://www.postgresql.org/docs/9.6/static/functions-datetime.html))
+  
 
diff --git a/DependencyInjection/ChillMainExtension.php b/DependencyInjection/ChillMainExtension.php
index 27449454b..8d7e1e202 100644
--- a/DependencyInjection/ChillMainExtension.php
+++ b/DependencyInjection/ChillMainExtension.php
@@ -30,6 +30,8 @@ use Chill\MainBundle\Doctrine\DQL\GetJsonFieldByKey;
 use Chill\MainBundle\Doctrine\DQL\Unaccent;
 use Chill\MainBundle\Doctrine\DQL\JsonAggregate;
 use Chill\MainBundle\Doctrine\DQL\JsonbExistsInArray;
+use Chill\MainBundle\Doctrine\DQL\Similarity;
+use Chill\MainBundle\Doctrine\DQL\OverlapsI;
 
 /**
  * This class load config for chillMainExtension.
@@ -159,7 +161,9 @@ class ChillMainExtension extends Extension implements PrependExtensionInterface,
                     'AGGREGATE' => JsonAggregate::class
                  ),
                  'numeric_functions' => [
-                    'JSONB_EXISTS_IN_ARRAY' => JsonbExistsInArray::class
+                    'JSONB_EXISTS_IN_ARRAY' => JsonbExistsInArray::class,
+                    'SIMILARITY' => Similarity::class,
+                    'OVERLAPSI' => OverlapsI::class
                 ]
               )
            )
diff --git a/Doctrine/DQL/OverlapsI.php b/Doctrine/DQL/OverlapsI.php
new file mode 100644
index 000000000..ae9df72f4
--- /dev/null
+++ b/Doctrine/DQL/OverlapsI.php
@@ -0,0 +1,110 @@
+<?php
+/*
+ * Chill is a software for social workers
+ * Copyright (C) 2018 Champs-Libres Coopérative <info@champs-libres.coop>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+namespace Chill\MainBundle\Doctrine\DQL;
+
+use Doctrine\ORM\Query\AST\Functions\FunctionNode;
+use Doctrine\ORM\Query\Lexer;
+
+/**
+ * DQL function for OVERLAPS function in postgresql
+ * 
+ * If a value is null in period start, it will be replaced by -infinity. 
+ * If a value is null in period end, it will be replaced by infinity
+ * 
+ */
+class OverlapsI extends FunctionNode
+{
+    private $firstPeriodStart;
+    
+    private $firstPeriodEnd;
+    
+    private $secondPeriodStart;
+    
+    private $secondPeriodEnd;
+    
+    public function getSql(\Doctrine\ORM\Query\SqlWalker $sqlWalker)
+    {
+        return '('
+            .$this->makeCase($sqlWalker, $this->firstPeriodStart, 'start').', '
+            .$this->makeCase($sqlWalker, $this->firstPeriodEnd, 'end').
+            ') OVERLAPS ('
+            .$this->makeCase($sqlWalker, $this->secondPeriodStart, 'start').', '
+            .$this->makeCase($sqlWalker, $this->secondPeriodEnd, 'end').')'
+            ;
+    }
+    
+    protected function makeCase($sqlWalker, $part, $position)
+    {
+        //return $part->dispatch($sqlWalker);
+        
+        switch ($position) {
+            case 'start' :
+                $p = '-infinity';
+                break;
+            case 'end':
+                $p = 'infinity';
+                break;
+        }
+        
+        if ($part instanceof \Doctrine\ORM\Query\AST\PathExpression) {
+            return 'CASE WHEN '
+                .'  '.$part->dispatch($sqlWalker).' IS NOT NULL '
+                . 'THEN '.
+                $part->dispatch($sqlWalker)
+                . ' ELSE '.
+                "'".$p."'::date "
+                . 'END';
+        } else {
+            return 'CASE WHEN '
+                .'  '.$part->dispatch($sqlWalker).'::date IS NOT NULL '
+                . 'THEN '.
+                $part->dispatch($sqlWalker)
+                . '::date ELSE '.
+                "'".$p."'::date "
+                . 'END';
+        }
+    }
+
+    public function parse(\Doctrine\ORM\Query\Parser $parser)
+    {
+        $parser->match(Lexer::T_IDENTIFIER);
+        
+        $parser->match(Lexer::T_OPEN_PARENTHESIS);
+        
+        $this->firstPeriodStart = $parser->StringPrimary();
+        
+        $parser->match(Lexer::T_COMMA);
+        
+        $this->firstPeriodEnd = $parser->StringPrimary();
+        
+        $parser->match(Lexer::T_CLOSE_PARENTHESIS);
+        
+        $parser->match(Lexer::T_COMMA);
+        
+        $parser->match(Lexer::T_OPEN_PARENTHESIS);
+        
+        $this->secondPeriodStart = $parser->StringPrimary();
+        
+        $parser->match(Lexer::T_COMMA);
+        
+        $this->secondPeriodEnd = $parser->StringPrimary();
+        
+        $parser->match(Lexer::T_CLOSE_PARENTHESIS);
+    }
+}
diff --git a/Doctrine/DQL/Similarity.php b/Doctrine/DQL/Similarity.php
new file mode 100644
index 000000000..988e55f5f
--- /dev/null
+++ b/Doctrine/DQL/Similarity.php
@@ -0,0 +1,54 @@
+<?php
+/*
+ * Chill is a software for social workers
+ * Copyright (C) 2018 Champs-Libres Coopérative <info@champs-libres.coop>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+namespace Chill\MainBundle\Doctrine\DQL;
+
+use Doctrine\ORM\Query\AST\Functions\FunctionNode;
+use Doctrine\ORM\Query\Lexer;
+
+/**
+ * 
+ *
+ * 
+ */
+class Similarity extends FunctionNode
+{
+    private $firstPart;
+    
+    private $secondPart;
+    
+    public function getSql(\Doctrine\ORM\Query\SqlWalker $sqlWalker)
+    {
+        return 'SIMILARITY('.$this->firstPart->dispatch($sqlWalker). 
+            ', ' . $this->secondPart->dispatch($sqlWalker) .")";
+    }
+
+    public function parse(\Doctrine\ORM\Query\Parser $parser)
+    {
+        $parser->match(Lexer::T_IDENTIFIER);
+        $parser->match(Lexer::T_OPEN_PARENTHESIS);
+        
+        $this->firstPart = $parser->StringPrimary();
+        
+        $parser->match(Lexer::T_COMMA);
+        
+        $this->secondPart = $parser->StringPrimary();
+        
+        $parser->match(Lexer::T_CLOSE_PARENTHESIS);
+    }
+}