diff --git a/labellines/core.py b/labellines/core.py
index 011a4d4..5b32e13 100644
--- a/labellines/core.py
+++ b/labellines/core.py
@@ -283,11 +283,22 @@ def labelLines(
     else:
         converter = ax.xaxis.get_converter()
     time_classes = (_SwitchableDateConverter, DateConverter, ConciseDateConverter)
-    if isinstance(converter, time_classes):
-        xvals = [
-            num2date(x).replace(tzinfo=ax.xaxis.get_units())
-            for x in xvals  # type: ignore
-        ]
+    if xvals is None:
+        raise ValueError(
+            "xvals must be a tuple of two floats or a list of floats."
+            f"Got {xvals} instead."
+        )
+    elif isinstance(converter, time_classes):
+        xvals_dates = []
+        for x in xvals:
+            if isinstance(x, datetime):
+                x_datetime = x
+            elif isinstance(x, np.datetime64):
+                x_datetime = x.astype(datetime)
+            else:
+                x_datetime = num2date(x)
+            xvals_dates.append(x_datetime.replace(tzinfo=ax.xaxis.get_units()))
+        xvals = xvals_dates
 
     txts = []
     try: