How to make dynamic system call in sketch

I'd like to make system() calls in Arduino sketches with dynamic values.

  String dateCmd="date +'%Y%m%d %H:%M:%S' -s ";

  dateCmd+="'20150120 10:42:00'";

  system(dateCmd);

However it generates an error:

error: cannot convert 'String' to 'const char*' for argument '1' to 'int system(const char*)'

I've tried to use c_str() method of String, but it doesn't exist.

What shoud I do to get it to work?