Pruning time?

Comment from CLIVE JONES

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I need to prune a lilac which hasn't been properly pruned for many years and what few flowers it produced were at the top. It is now about 8 - 9 ft high with no laterals below 2.5 ft. The main stems at the base are about 2 ins diam. I think I should be hard pruning this but would like to know when is the best time? Photo attached.