@peter.medbury It’s not exactly what you’re looking for but it might be useful in your quest.
I’m using it to calculate the days ago for blog posts http://prntscr.com/mybpup
{% comment %} convert our dates to Number of seconds since 1970-01-01 00:00:00 UTC {% endcomment %}
{% assign dateStart = this['ReleaseDate'] | date: '%s' %}
{% assign nowTimestamp = 'now' | date: '%s' %}
{% comment %} difference in seconds {% endcomment %}
{% assign diffSeconds = nowTimestamp | minus: dateStart %}
{% comment %} difference in days {% endcomment %}
{% assign diffDays = diffSeconds | divided_by: 3600 | divided_by: 24 %}
{% assign diffMonths = diffSeconds | divided_by: 3600 | divided_by: 24 | divided_by: 30.42 | round %}
{% if diffDays > 365 %}
{{ diffMonths | divided_by: 12 | round }} year{% if diffMonths > 12 %}s{% endif %} ago
{% elsif diffDays > 30.42 %}
{{ diffMonths }} month{% if diffMonths > 1 %}s{% endif %} ago
{% else %}
{{ diffDays | round }} day{% if diffDays > 1 %}s{% endif %} ago {% endif %} <span> {{this['ReleaseDate'] | date: " %b %d, %Y"}}<span> </div>