The current implementation of the "Date/Time" value doesn't facilitate the use of a variable to add or subtract time. In addition, it represents a specific point in time, not a timespan.
Without the requirement for a variable setting, you could use the "Add" option of the "Set a Date/Time Value" action to add 15 seconds to the start time, and check whether it's Less than the current date/time, as you're doing in your example.
Otherwise, you'll have to use the "{DATETICKS}" token and decimal values:
Begin Boolean Compare : [Cloak_State] Equals False
Set decimal [CD_Fin] value to the value of [ExeTime_Cloak]
Set decimal [CD_Fin] to [CD_Fin] plus [CD_Cloak]
Begin Decimal Compare : [{DATETICKS}] Is Greater Than [CD_Fin]
Say, 'Cloaking'
Press Tab key and hold for 0.1 seconds and release
Set Boolean [Cloak_State] to True
Else
Say, 'Cloak still on cooldown'
End Condition
Else
Say, 'Decloaking'
Press Tab key and hold for 0.1 seconds and release
Set decimal [ExeTime_Cloak] value to the converted value of {DATETICKS}
Set Boolean [Cloak_State] to False
End Condition
Where "CD_Cloak" has a value of 150000000 (One second is 10,000,000 ticks).
You could also use "{EXP:}" to set the value of CD_Fin to do it in one action instead of two:
Set decimal [CD_Fin] value to the value of [{EXP: {DEC:ExeTime_Cloak} + {DEC:CD_Cloak}}]
However, this may not work with all system locales.