Just assign the current date to a variable in awk and use this variable in the printf output, e.g.:
iwinfo wlan0 assoclist | awk -v date="$(date "+%d.%m.%Y %H:%M:%S")" '/^[[:alnum:]]/{printf "%s ::: %s",date,$0;for(line=1;line<=3;line++){getline;printf(line==3)?$0"\n":$0}}'
Output:
01.01.2020 19:53:25 ::: AA:BB:CC:DD:EE:FF -69 dBm / unknown (SNR -69) 0 ms ago RX: 108.0 MBit/s 19581 Pkts. TX: 121.5 MBit/s 39076 Pkts. expected throughput: unknown
Edit: Happy New Year! 