Don't do it.
Store dates in the DB as DateTime, and use SQlCommand.Parameters.AddWithValue to add the DateTime.Now directly, rather than converting it to a string in the first place.
Additionally:
Don't do the convert like that anyway!
1) When dealing with DateTime.Now, get a single instance and work from that. Each time you get DateTime.Now, you get the current date and time - this can lead to odd bugs near midnight and especially near Jan 1st!
DateTime now = DateTime.Now;
string s = now.ToString("yyyyMMdd");
...
2) Don't do the conversion in multiple lines. If you use a single DateTime.ToString it will be more efficient, and work better as you will have better control over the string produced. There is a full list of ToString formats here:
Formatting a DateTime for display - format string description[
^] which should help.
If you change as suggested, your problem will disappear.