How to convert an epoch time in millisecond into an ISO like string according to local time?

Solution 1:

If your system/environment implements the placeholder %N for nanoseconds, you could use %3N to get the padded milliseconds:

TZ=America/New_York jq -r '
  
  def mydate:
    ./1000 | strflocaltime("%Y-%m-%d %H:%M:%S.%3N");
  
  mydate

' <<< 1642812926681

2022-01-21 19:55:26.681

No Demo (jqplay.org does not support the %N placeholder)

If it doesn't, you could still do some tweaks to shorten it:

omit calculating the floor for the whole seconds as strflocaltime will take care of it
replace %Y-%m-%d %H:%M:%S with %F %T
use string interpolation "\(…)" for easier string conversion

TZ=America/New_York jq -r '
  
  def mydate:
    (./1000 | strflocaltime("%F %T.")) + "00\(.%1000)"[-3:];
  
  mydate

' <<< 1642812926681

2022-01-21 19:55:26.681

Demo