Are there really 93 months to act?

Communicating the urgency of climate change is an ongoing challenge. The problem is the time lag between GHG emissions and the climate impact, what we often refer to as "committed warming". There's also a "societal" warming commitment, caused by the lag between a decision to control emissions and the actual emissions control. That's why Jim Hansen protests the construction of new coal-fired power plants: they are a promise of future emissions and thus increase the societal warming commitment.

The latest approach is to provide estimates of how much time we have "left" to avoid dangerous climate change. Take Andrew Simms in the Guardian (please?):

Whatever the mistakes that allowed this situation to arise, there is growing international consensus that the best way out is via a green new deal policy package. Parts of the UK economy are in freefall with unemployment rising rapidly. At the same time, with less than 100 months to go before the world enters a new, more dangerous phase of global warming, there is an urgent need for the rapid environmental transformation of the economy.

100 months? I don't recall that in the IPCC. Simms' evidence is summarised here and here - there's even a ticking clock, a la the Fox TV show 24.

We found that, given all of the above, 100 months from today we will reach a concentration of greenhouse gases at which it is no longer "likely" that we will stay below the 2C temperature rise threshold. "Likely" in this context refers to the definition of risk used by the IPCC. But, even just before that point, there is still a one third chance of crossing the line.

Is this a useful PR tool? Or, in the desperation to encourage action on climate change, are the "100 hundred months" crowd causing more harm by providing false certainty in future predictions? For example, what will happen in January, 2017 if the world has not begun seriously curbing emissions -- or if it has?