diff --git a/draftlogs/6751_fix.md b/draftlogs/6751_fix.md
new file mode 100644
index 00000000000..824f4006de2
--- /dev/null
+++ b/draftlogs/6751_fix.md
@@ -0,0 +1 @@
+-  Timestamp '1970-01-01 00:00:00' formats hoverformat '%H:%M:%S' as 'undefined' [[#6752](https://github.com/plotly/plotly.js/pull/6752)]
\ No newline at end of file
diff --git a/src/plots/cartesian/axes.js b/src/plots/cartesian/axes.js
index d3a56d04ad3..29627824766 100644
--- a/src/plots/cartesian/axes.js
+++ b/src/plots/cartesian/axes.js
@@ -1744,6 +1744,9 @@ function formatDate(ax, out, hover, extraPrecision) {
     var tr = ax._tickround;
     var fmt = (hover && ax.hoverformat) || axes.getTickFormat(ax);
 
+    // Only apply extra precision if no explicit format was provided.
+    extraPrecision = !fmt && extraPrecision;
+
     if(extraPrecision) {
         // second or sub-second precision: extra always shows max digits.
         // for other fields, extra precision just adds one field.
@@ -1768,7 +1771,7 @@ function formatDate(ax, out, hover, extraPrecision) {
         // anything to be uniform with!)
 
         // can we remove the whole time part?
-        if(dateStr === '00:00:00' || dateStr === '00:00') {
+        if(headStr !== undefined && (dateStr === '00:00:00' || dateStr === '00:00')) {
             dateStr = headStr;
             headStr = '';
         } else if(dateStr.length === 8) {