Gem costs increases with time you want to reduce.
For the first 6 minutes you need 1 gem. Every further 3 minutes it costs 1 gem additionly. So first hour costs 19 gems until 59m and 59 sec.
While complete first hour costs 20 gems, every further hour of the first day only costs 10 gems additionly.
So 23h and 59 min cost 249 gems. Complete first day costs 252 gems (added 3 instead of 1 gem for last second of the first day).
While complete first day so costs 252 gems, every further day only costs 120 gems (5 per hour) additionly.
|Time to reduce||Gems needed||Difference|
|< 9m||2||1 per 3 min|
|< 2h||29||10 per hour|
|2d||372||120 per day|