The last cooler than average month was in February 1994
Here's the tweet:
That's it. That's the whole tweet. For additional context, this data comes from the HadCRUT4 dataset, a global temperature dataset compiled by the Hadley Centre of the UK Met Office and the Climatic Research Unit (CRU) of the University of East Anglia. Both are funded by The National Centre for Atmospheric Science in the UK.
You can download a similar graph in a different format here. Here's what that graph looks like: