We do have those. In this case the common library is called "libc." In this case (and in a few others) the Go team decided to intentionally avoid providing an interface to this function.
The good news is it would take just a few minutes to add support for go to call strftime directly from libc.
Besides, users of a library in language X don't want to have to read language Y to look under the hood.
There are some mature languages with an active "modding" community. Haskell, for example.
Which we have. Several groups have done this. eg http://docs.paralleluniverse.co/quasar/
No, you don't. You can use one of the 30 existing ways of formatting dates from other languages. You might have to implement it yourself but you don't have to design it yourself.
This is a symptom of the main problem with Go. Its creators either don't know about or have chosen to ignore most of the last few decades of language design and do their own thing when it doesn't add anything significant. And no, they don't do concurrency differently: coroutines have been around for a long time.
Unfortunately there are also some downsides to the virtual machine approach but I still think there's a window of opportunity open here for Microsoft if they make the CLR robust and reliable on Mac and Linux.