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Basic Mechanics
What is Expiration Rate?
The Expiration Rate of an effect is the rate at which time passes relative to other mechanics. For most effects, the default Expiration Rate is 1, meaning that time passes as normal for that effect.
Whilst modifiers to the Expiration Rate of an effect will make the effect last longer or end sooner, the actual Duration remains unchanged. Expiration Rate modifiers are calculated separately and follow different rules, and only alter the effective Duration of an effect.
Note: Expiration Rate modifiers are not the same as Duration scalers and are calculated separately.
Calculating Expiration Rate
There are two formulas that can be used to calculate the Expiration Rate of an effect and its impact on the effective Duration of that effect.
Expiration Rate = 1 + X / 100 - Y / 100
Where X = the sum of all relevant Expire % faster modifiers and Y = the sum of all relevant Expire % slower modifiers
Effective Duration = A / B
Where A = the base Duration of an effect and B = the Expiration Rate of that effect
Example calculation:
A character has allocated 
Lasting Boons on the Passive Skill Tree and has Instilled 
Prolonged Assault on their Amulet. This character gains the Arcane Surge buff from Arcane Surge Support, lasting a base Duration of 10 seconds. The character has a total of 20% slower Expiration Rate of buffs, and no modifiers to faster Expiration Rate, making their final Expiration Rate for buffs 0.8 (1 + 0 - 0.2). From this Expiration Rate we can calculate the effective Duration of the Arcane Surge buff, which is 12.5 seconds (10 / 0.8).
Note: modifiers to Faster Expiration Rate have diminishing returns, whilst modifiers to Slower Expiration Rate have increasing returns.
Expiration Rate Cap
The slower Expiration Rate of an effect is capped at 75% from all sources combined, for an effective Duration of four times the base Duration of that effect ( 1 / 0.25 = 4).