从datetime2数据类型到datetime数据类型的转换导致值超出范围

You shouldn't think about the database as storing the values in a particular string format at all - any more than it stores numbers as decimal or hex.

Instead, you should just view it as a date/time, e.g.

// TODO: Do you really want the local time, rather than UtcNow?
// TODO: Change TRANSACT_DATE to be TransactionDate ideally, to follow
// .NET naming conventions
newEntry.TRANSACT_DATE = DateTime.Now;

When you retrieve the value, you should get a DateTime at that point too. Then if you want to display the value to the user you can apply a particular format. Different users may well want different formats - and may even want to display the same date/time in different time zones.

It's important to differentiate between the intrinsic data you store (in this case a date/time) and the text format that happens to be used to display it any one particular context/application. You should avoid conversions to/from strings at any time when you don't really need one. Ideally, these should only be at the boundaries of your application - e.g. when displaying text to a user, or potentially serializing to JSON or XML. Whenever an API allows you to not perform a conversion (e.g. with a database parameter) you should avoid it.

至于您当前的错误-是否有可能是您未填充的其他字段，因此正在使用default(DateTime)，这将超出范围吗？这很有道理-但DateTime.Now实际上不应超出范围，除非您已应用了单独的约束，或者您的系统时钟距离很远。