'Acche din' slogan was originally coined by Manmohan Singh:

Prime Minister Narendra Modi's famous slogan of 'acche din' had originally been coined by his predecessor Manmohan Singh but has now become the "millstone" around the NDA government's neck, Union Minister Nitin Gadkari said today.
"Acche din maanne se hota hai (Good days depends on one's belief). It was at an NRI meet in Delhi that Manmohan Singh had said that 'Acche din ayenge' (good days will come).
"When asked when 'acche din' will come, Singh had replied, 'in the future'. Modiji said the same thing somewhere and it has now become the millstone around our neck," Gadkari said at an event here.

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"Achhe din kabhi nahi aate. Achhe din ke naare gale mein fasi haddi hain (Good days never come. The slogan of Achhe din is a bone stuck in the throat for the National Democratic Alliance government," Gadkari said.
Responding to a query by former MP Vijay Darda, the minister quipped, "We just used the words 'Achhe Din' and it should not be taken in the literal sense. It should be interpreted as progress is being made."
Gadkari said, "If a person has a bicycle, he would want a motorcycle, once the motorcycle is bought, the next target would be a car. Therefore, no one ever feels that good days have arrived.