Why?
DateTime.Now is already a DateTime value ... so why convert that to a string, just to convert it back to a DateTime - especially as you specify a format to the ToString but use the system default for the Convert?
Just use DateTime.Now directly:
DateTime now = DateTime.Now;
string formattedDate = $"Report -{now.Day:00}-{now.Month:00}-{now.Hour:00}-{now.Minute:00}-{now.Second:00}";
string fileSavePath = Path.Combine(folderPath, formattedDate + ".xlsx");
Note: I copy DateTime.Now into a variable - this is important as multiple reads of DateTime.Now can give consistency problems around midnight!