If you're happy with that solution, then you don't need to read my alternate solution.
There's a way to convert some "natural language time expressions" to "standard date expressions" using the "at" command in Unix, which MacOS supports (but it currently deprecated.) You don't need Javascript for this at all.
For example, the standard Unix command "at midnight next month" will let you enter a command to be executed at the first midnight next month. The result of the command will be a standard date format string. Here's a table I found online that lists SOME (but not all) of the natural language dates you can specify: (yay! I managed to insert my first table successfully on this website)
| Time Expression | Time of Command Execution |
|---|---|
| noon | 12:00 PM November 18 2020 |
| midnight | 12:00 AM November 19 2020 |
| tomorrow | 8:00 AM November 19 2020 |
| noon tomorrow | 12:00 PM November 19 2020 |
| next week | 8:00 AM November 25 2020 |
| next Monday | 8:00 AM November 23 2020 |
| fri | 8:00 AM November 20 2020 |
| 6:00 AM | 6:00 AM November 19 2020 |
| 2:30 PM | 2:30 PM November 18 2020 |
| 1430 | 2:30 PM November 18 2020 |
| 2:30 PM tomorrow | 2:30 PM November 19 2020 |
| 2:30 PM next month | 2:30 PM December 18 2020 |
| 2:30 PM 11/21 | 2:30 PM November 21 2020 |
| 2:30 PM Nov 21 | 2:30 PM November 21 2020 |
| 2:30 PM 11/21/2020 | 2:30 PM November 21 2020 |
| now + 30 minutes | 8:30 AM November 18 2020 |
| now + 1 hour | 9:00 AM November 18 2020 |
| now + 2 days | 8:00 AM November 20 2020 |
| 4 PM + 2 days | 4:00 PM November 20 2020 |
| now + 3 weeks | 8:00 AM December 8 2020 |
| now + 4 months | 8:00 AM March 18 2021 |
Here's an actual example of the command in action. Right now the time is about noon on my Mac.
~/ at teatime + 30 minutes
say nothing
job 22 at Sat Feb 19 16:30:00 2022
Notice how it produced an actual date-time string from the words "teatime + 30 minutes".
If you don't need this sort of feature, feel free to ignore this post.