You may want to know date of 6 weeks from now:
$ weeks-from-now 6
2017-09-19
You may even want to know date of 3 weeks ago:
$ weeks-from-now 3 --past
2017-07-18
Following is the most easy way to install the tool (assuming you have the Rust toolchain installed):
cargo install weeks-from-now
NOTE: minimum required rustc is v1.64, due to clap.