The curious aspect is that for:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("-u", "--utc", choices=["-1"])
args = parser.parse_args()

it works. But if we use the colon (:) character as:
import argparse
parser = argparse.ArgumentParser()
parser.add_argument("-u", "--utc", choices=["-1:"])
args = parser.parse_args()

it does not work anymore... I could think maybe the issue could be related to the presence of ":" character together with "-" character.

For example for:
choices=["-:"] does not work
choices=["-"] works
choices=[":"] works
choices=[":-"] works

So, we get the error if the option start with "-" and has inside also ":" character. At this point I don't think it is related strictly to "negative numbers".