Hi @Harun Ergün,
You can try this:
DateTime? dt = string.IsNullOrEmpty(date) ? (DateTime?)null : DateTime.Parse(date);
It is not clear how you save the data to the database, you can refer to the following to add
cmd.Parameters.AddWithValue("@Date", (object)dt ?? DBNull.Value);
If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.