I have now discovered the issue with my code (or PHP7).
It turns out that the WordPress SVN now lints code with PHP7, after installing PHP7 on my testing machine and linting the code it turns out that the function getMonth() was causing the issue due to the numeric literals being prefixed with '0'
By enclosing the numbers in quotation marks the issue has been resolved.
e.g. changing:
if ($month == 01)
{
return 'Jan';
}
to:
if ($month == '01')
{
return 'Jan';
}