fix:gant chart crashing in browser

on-behalf-of: @Mermaid-Chart <hello@mermaidchart.com>

cypress/integration/rendering/gantt.spec.js

@@ -803,4 +803,34 @@ describe('Gantt diagram', () => {
       {}
     );
   });
+  it('should handle numeric timestamps with dateFormat x', () => {
+    imgSnapshotTest(
+      `
+     gantt
+     title Process time profile (ms)
+     dateFormat x
+     axisFormat %L
+     tickInterval 250millisecond
+
+     section Pipeline
+     Parse JSON p1: 000, 120
+    `,
+      {}
+    );
+  });
+  it('should handle numeric timestamps with dateFormat X', () => {
+    imgSnapshotTest(
+      `
+     gantt
+     title Process time profile (ms)
+     dateFormat X
+     axisFormat %L
+     tickInterval 250millisecond
+
+     section Pipeline
+     Parse JSON p1: 000, 120
+    `,
+      {}
+    );
+  });
 });

packages/mermaid/src/diagrams/gantt/ganttDb.js

@@ -268,7 +268,9 @@ const fixTaskDates = function (startTime, endTime, dateFormat, excludes, include
 
 const getStartDate = function (prevTime, dateFormat, str) {
   str = str.trim();
-
+  if ((dateFormat.trim() === 'x' || dateFormat.trim() === 'X') && /^\d+$/.test(str)) {
+    return new Date(Number(str));
+  }
   // Test for after
   const afterRePattern = /^after\s+(?<ids>[\d\w- ]+)/;
   const afterStatement = afterRePattern.exec(str);