Duration must also cope with Google Sheets export of cells having a formula. Rather than exporting the native value, they export the formatted value.

EPPlus/ExcelWorksheet.cs

diff --git a/EPPlus/ExcelWorksheet.cs b/EPPlus/ExcelWorksheet.cs
index e4f001e..64c0899 100644
--- a/EPPlus/ExcelWorksheet.cs
+++ b/EPPlus/ExcelWorksheet.cs
@@ -1496,12 +1496,16 @@
                         }
                         else
                         {
-                            _values.SetValue(row, col, "");
+                            // Cope with Google Sheets export of cells having a formula.
+                            // Rather than exporting the native value, they export the formatted value.
+                            _values.SetValue(row, col, v);
                         } 
                     }
                     else
                     {
-                        _values.SetValue(row, col, "");
+                        // Cope with Google Sheets export of cells having a formula.
+                        // Rather than exporting the native value, they export the formatted value.
+                        _values.SetValue(row, col, v);
                     }
                 }
                 else if ((nf >= 14 && nf <= 19) || (nf == 22))   // DateTime
@@ -1519,12 +1523,16 @@
                         }
                         else
                         {
-                            _values.SetValue(row, col, "");
+                            // Cope with Google Sheets export of cells having a formula.
+                            // Rather than exporting the native value, they export the formatted value.
+                            _values.SetValue(row, col, v);
                         }
                     }
                     else
                     {
-                        _values.SetValue(row, col, "");
+                        // Cope with Google Sheets export of cells having a formula.
+                        // Rather than exporting the native value, they export the formatted value.
+                        _values.SetValue(row, col, v);
                     }
                 }
                 else