The installer information on the recovery server is damaged

And how do I trash the install macOS Sierra. app, coz it doesn’t let me select it and all I can access is the OS utility menu which has four options -

1. reinstall from time machine back up (something like that)
2. reinstall a new copy of OS
3. online help
4. disk utility

It’s connected through WiFi and no I don’t have a USB to Ethernet cable neither a thunderbolt to Ethernet cable, should I buy it, will it work 🤔😊

I tried changing the date with the WiFi or network off but when I do that through terminal it gives me an error, bad command but then again when I turn on the WiFi or network it accepts the same command and the date changes, and then when I try to install the OS Sierra it gives me a different error message “cer_tile_untrusted”

command I use to change date is -

date m/date/time/year

when I first switch on the MacBook Air there is nothing on the screen except a folder with a question mark and so when I reboot the MacBook holding down any of the combination keys like option + R or cmd + option + R it takes me to the internet recovery and after waiting for like 40 minutes to 1 hour when the download is complete I get a language chose option so when I proceed it takes me to OS Utility where I try and reinstall a new copy of OS from the utility menu it takes me to the OS Sierra page, when I click continue it verifies, after verifying and then bam comes the message “the installer information on the recovery server is damaged”

I tried but it’s not working, the date changed on the offline mode, but then again to install it needs to go online and so again the message is prompted “cert_title_untrusted” and when I change back the date to the present it gives a different message “the installer on the recovery server is damaged”, I m really confused as to what to do

Thank u all 🙏 for the suggestions, finally it worked and now my MacBook is good and working. Thank u