It's a feature that when specified with a date (using Epoch Converter) or block height (the block number of a future block),
that transaction wont be mined and/or will be rejected by most nodes by default until that time/block has passed.

For normal transactions: it's just 0 or the current block height.

I believe that uses a CLTV script to make a "time-locked address", not totally just nLocktime.
@Abiky It makes the outputs (received bitcoins) unspendable until the specified date/block, it's more useful than just nLockTime parameter and fits perfectly to your usage description in the OP.