Unfortunately, due to protocolbuffers/protobuf#21301, including time_util inside of a header will cause a lot of headaches on MSVC/Windows. While that can and should be resolved separately of this, in the interim there will be a lot of people using protobuf versions without a fix for this for the foreseeable future. This leaves us with only a couple of obvious options: either inline the constants, or move the function definitions into time.cc. I chose the latter since it doesn't seem like there is likely to be a substantial performance hit for this, as these are only used in a couple of locations that don't appear to be very hot paths.