DD-WRT (Busybox Linux)
Here is a command-line that uses curl to get the current time from the HTTP response header:
* Get current time in Epoch/Unix format by curl-ing Google, using sed and grep to filter out irrelevant information:
curl -I 'http://www.google.com/' 2>/dev/null | grep -i '^date:' | sed 's/^[Dd]ate: //g'
Result: Fri, 09 Feb 2018 08:58:49 GMT
* Convert the Unix time string into YYYYMMDDHHMM.SS using awk
curl -I 'http://www.google.com/' 2>/dev/null | grep -i '^date:' | sed 's/^[Dd]ate: //g' | awk 'BEGIN{months="JanFebMarAprMayJunJulAugSepOctNovDec"}{printf("%s%02d%s%s", $4, index(months,$3)/3+1,$2,$5)}' | sed 's/://' | sed 's/:/./'
Result: 201802090858.49
* Set system date by calling "date --utc -s" with "`curl .....`" as parameter:
date --utc -s `curl -I 'http://www.google.com/' 2>/dev/null | grep -i '^date:' | sed 's/^[Dd]ate: //g' | awk 'BEGIN{months="JanFebMarAprMayJunJulAugSepOctNovDec"}{printf("%s%02d%s%s", $4, index(months,$3)/3+1,$2,$5)}' | sed 's/://' | sed 's/:/./'`
If you don't trust Google for time, maybe you could try http://www.usno.navy.mil/USNO/time. I believe it's the master clock for everything in USA. Well, I don't know the details though.
BTW, the above script was also listed in DD-WRT's forum:
New Build 34876 (BS): 02-08-2018-r34876