Missing timestamp implementation on rpi3

[bob prohaska]

Efforts to compile www/firefox on an rpi3 running -current have gotten
to the point where make reports

mozbuild.frontend.reader.BuildReaderError: 
==============================
FATAL ERROR PROCESSING MOZBUILD FILE
==============================

The error occurred while processing the following file:

    /usr/ports/www/firefox/work/firefox-65.0.2/mozglue/misc/moz.build

A moz.build file called the error() function.

The error it encountered is:

    No TimeStamp implementation on this platform.  Build will not succeed

Correct the error condition and try again.

It isn't obvious whether this is a problem with the port or the system,
thus the cross-post. Any guidance appreciated.

Thanks for reading,

bob prohaska