diff --git a/Operand.cpp b/Operand.cpp
index ca4c94f..db245c2 100644
--- a/Operand.cpp
+++ b/Operand.cpp
@@ -404,11 +404,28 @@ namespace jet {
 	   throw coreutils::Exception("Expecting , in DATEDIFF expression.");
 	 Operand parm2(in, tag);
 	 if(in.ifNext(")")) {
-
-	    // TODO: Implement DATEDIFF.
-
+	    // Matches MySQL's DATEDIFF(expr1, expr2): expr1 - expr2 expressed
+	    // as a whole number of days, ignoring any time-of-day component.
+	    // Dates are parsed as YYYY-MM-DD (a leading date portion of a
+	    // longer "YYYY-MM-DD HH:MM:SS" value parses fine too, since
+	    // strptime only consumes what the format string asks for) and
+	    // interpreted as UTC via timegm() (not mktime(), which is
+	    // local-timezone-dependent) so the result doesn't shift with the
+	    // server's own timezone setting.
+	    struct tm tm1 = {};
+	    struct tm tm2 = {};
+	    if(!strptime(parm1.string.c_str(), "%Y-%m-%d", &tm1))
+	      throw coreutils::Exception("DATEDIFF: first argument is not a valid date (expected YYYY-MM-DD).");
+	    if(!strptime(parm2.string.c_str(), "%Y-%m-%d", &tm2))
+	      throw coreutils::Exception("DATEDIFF: second argument is not a valid date (expected YYYY-MM-DD).");
+	    time_t t1 = timegm(&tm1);
+	    time_t t2 = timegm(&tm2);
+	    doubleValue = difftime(t1, t2) / 86400.0;
+	    isNumber = true;
+	    string = std::format("{:.12f}", doubleValue);
+	    string.removeTrailingZeros();
 	 } else
-	   throw coreutils::Exception("Expecting ) at end of DATEDIFF expression.");	 	       
+	   throw coreutils::Exception("Expecting ) at end of DATEDIFF expression.");
       } else if(in.ifNextIgnoreCase("true")) {
 	 boolean = true;
 	 string = "true";