Schedule this month one absolutely fixed (3,2,5)

I believe the answer is:

set in stone

'absolutely fixed' is the definition.

'schedule this month one' is the wordplay.
'schedule' becomes 'set'.
'this month' becomes 'inst' (letter-writing abbreviation for this month).
'set'+'inst'+'one'='SET IN STONE'

(Other definitions for set in stone that I've seen before include "Fixed and unalterable" , "As inflexible" , "as Excalibur?" .)

I've seen this clue in The Times.

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