You can use the time() function from stdlib, to make this a little less
resource intensive:
https://github.com/puppetlabs/puppetlabs-stdlib/blob/master/lib/puppet/parser/functions/time.rb
A different approach would be to use strftime (also from stdlib) to
interpolate the current year into the source URL of your cert and have
them called 'cert-2015.pem' and 'cert-2016.pem'
Felix' comment should be heeded, though. Especially the latter version
will "cause" an outage at the most inconvenient time of the year:
2016-01-01T00:10.
Cheers, David