“Never put a date and a number in the same sentence” (or “Never put a number and a date in the same sentence”) is a saying used both in business and politics. If an economic forecaster or a politician is specific with a number and a date, the promise or forecast will be remembered and, perhaps, come back to haunt the forecaster or politician when that date arrives.
 
British politican Harold Wilson (1916-1995) was credited with the saying in 1988, but it’s uncertain when he said it (or if he said it first).
 
   
Wikipedia: Harold Wilson
James Harold Wilson, Baron Wilson of Rievaulx, KG, OBE, PC, FRS, FSS (11 March 1916 – 24 May 1995) was a British Labour Party politician who served as the Prime Minister of the United Kingdom from 1964 to 1970 and 1974 to 1976.
     
9 August 1988, Boston (MA) Globe, “Bush Called ‘Kid in a Candy Store’” by Ben Bradlee Jr. and Thomas Oliphant, Globe Staff:
But citing a remark he attributed to former British Prime Minister Harold Wilson, who warned politicians never to put a date and a number in the same sentence, Dukakis, like Bush, refused to make public a detailed government spending proposal.
   
21 August 1988, Boston (MA) Globe, “At LBJ Ranch, Dukakis reaffirming he opposed involvement in Vietnam” by Ben Bradlee Jr., pg. 20:
At the press conference at the ranch, Dukakis declined comment on Bush’s assertion in his acceptance speech Thursday that he would create 30 million jobs over the next eight years, and he would not hazard a prediction on how many jobs his administration might create. Former British prime minister “Harold Wilson once said, ‘Never put a date and a number in the same sentence,’ ” Dukakis said. “I think it’s good advice . . .”
 
11 May 1994, Boston (MA) Globe, “Lawmakers, don’t duck the question,” pg. 18:
A smart politician, according to the old saying, never puts a number and a date in the same sentence.
 
Google Books
Rethinking Democratic Accountability
By Robert D. Behn
Washington, DC: Brookings Institution Press
2001
Pg. 15:
One governor told his department heads, “Never put a number and a date in the same sentence.” The message was clear: If you don’t promise to produce a certain result by a certain time, no one can hold you accountable for not doing so. Don’t overpromise. Don’t even promise. Don’t establish any public expectations for performance. Don’t give anyone the opportunity to hold you accountable for any kind of performance.
 
23 February 2004, Des Moines (IA) Business Record, “Economic Forecast” by SueAnn Jones, pg. 12:
You know, for our high school students, you know, some people ask, what’s the secret to being an effective economist? And I guess the answer is never put a number and a date in the same sentence. Always remember that. You want to last in this business; you don’t stay by being specific.
   
5 January 2011, Financial Times (FT.com), “Stock market forecasts”:
It is easy to make accurate financial forecasts - just avoid putting a number and a date in the same sentence. That old joke helps make sense of a rare moment of agreement between a famous pessimist and a group of Wall Street’s cheerleaders.
 
Twitter
GromovaSonya
‏@GromovaSonya
The secret of governmental success: “Never put a number and a date in the same sentence.”
3:30 AM - 7 Nov 2013
 
Twitter
Andy Langenkamp
‏@AndyLangenkamp
Note to forecasters: if you are forced to make a forecast, give a number and a date, but never in the same sentence http://ow.ly/spZjn
11:05 AM - 9 Jan 2014
 
International Banker
ASSESSING ECONOMIC RISK FACTORS
January 2, 2017
By Robert A. Dye, PhD, Chief Economist of Comerica Bank, Dallas, Texas
An old joke among economists is that to be an accurate forecaster, never give a number and a date together in the same sentence. I can tell you that GDP will increase by a 2.5 percent annual rate, but if I don’t tell you when, then I will likely be accurate at some point. The joke highlights a very humbling quality of economic forecasts; they are overwhelmingly likely to be wrong. The reason that they are so often wrong is that our forecasting models simply cannot account for all the levers and linkages and nonlinearities in the complex adaptive system that is a modern economy. When we build and use macroeconomic forecasting models, we are undertaking an exercise to reduce the degrees of freedom in an infinitely complex and ever-changing system. We reduce the complexity of the regional, country or global economy to a finite system with a fixed number of variables and fixed linkages between the variables. We make our forecasts, and then something else happens to push the trajectory of the economy into an unanticipated direction. So in addition to the skillsets required to build, maintain and operate forecasting models, economists and the users of forecasts need to develop skills in assessing the myriad of economic risk factors that will not be accounted for in their models.