Are you are using NSClient++ on your windows server ? If so you can find NSClient's documentation via the link below: https://nsclient.org/docs/
Unfortunately NSClient++ isn't our product and as a result we can't provide any official support or guidance for it. Have you tried using NCPA ?
If you are willing to try NCPA here is a link regarding information about it: https://www.nagios.org/ncpa/
I created a new check command in the core config manager. This is the command I created:
$USER1$/check_ncpa.py -H $HOSTADDRESS$ -t $_HOSTTOKEN$ -M $ARG1$ $ARG2$
From there I created a service using the check command I just created
$ARG1$: 'system/uptime'
$ARG2$: -c 2592000 -w 2419200
The warning is at the 28th day, while the critical is at the 30th day. You can change those (time in seconds) as you see fit for your alerting desires.
I then created a free variable in the host I wanted to monitor called _token and gave it the value of the NCPA token for that machine. If you only use one token for everything or only have one machine you can just put in your token instead of $_HOSTTOKEN$
If you have anymore questions please feel free to reach out
