Nemesis epoch time is hard coded in the SDK but can change on the server side.

For the mainnet release, the epochAdjustment value will be change.  This will require an SDK update.  Instead of hardcoded the SDK should read the value from the network config when available. 

[network]
epochAdjustment = 1573430400s

https://github.com/nemtech/symbol-sdk-typescript-javascript/blob/2b18ca2f991aa692dd4384e93b7377954faed695/src/model/transaction/Deadline.ts#L28