Solana Rust smart contracts have access to
solana_program::clock::Clock::get()?.unix_timestamp
which is seconds from epoch (midnight Jan 1st 1970 GMT) but has a significant drift from any real-world time-zone as a product of Solana's slowdowns over time. Many contracts factor in this unix timestamp when calculating reward amounts (notably Step Finance and therefore Gem Farm which reuses the logic). How can I reconstruct this Solana unix timestamp on the front-end in JavaScript without requiring any transaction / wallet signature? Calls to a Solana node RPC are fine.
You can use the getBlockTime endpoint from JSON RPC. First you'll need the highest slot using the getSlot. That would become:
const connection = new Connection('https://api.testnet.solana.com', 'processed');
const slot = await connection.getSlot();
const timestamp = await connection.getBlockTime(slot);
More info at https://docs.solana.com/developing/clients/jsonrpc-api#getblocktime and https://docs.solana.com/developing/clients/jsonrpc-api#getslot