Thanks to Valentino Vranken who provides this useful post: Using A .Net 3.5 Assembly In SSIS 2008
By default SSIS Script Tasks / Components reference .NET Framework 2.0, which was confusing me as I tried to implement a solution around managing times in different Time Zones which required the TimeZoneInfo Class which only exists in 3.5 and higher. (This was what I was trying to implement: Daylight Savings in Script Task). Because the script referenced .NET Framework 2.0 the TimeZoneInfo class wouldn’t work. Switch the target framework to 3.5 and bingo – all good.
The tl/dr version: In your script component – while editing the script – open the Properties and change the Target Framework to 3.5 to use functions available in 3.5.