How can I change the default date format (using LC_TIME)?

Question

How do I change the format of date command by modifying LC_TIME in locale?

Currently the day of month uses %e format. I need it to be displayed in %d format.

Below is the Current Format:

#date
Thu Aug 9 18:26:11 IST 2018

Expected format:

#date
Thu Aug 09 18:26:11 IST 2018

Here is my locale:

#locale
LANG=en_US.UTF-8
LANGUAGE=
LC_CTYPE=en_IN
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=

LC_TIME section in /usr/share/i18n/locales/en_US:

    LC_TIME
abday   "Sun";"Mon";"Tue";"Wed";"Thu";"Fri";"Sat"
day "Sunday";/
    "Monday";/
    "Tuesday";/
    "Wednesday";/
    "Thursday";/
    "Friday";/
    "Saturday"

week 7;19971130;1
abmon   "Jan";"Feb";/
    "Mar";"Apr";/
    "May";"Jun";/
    "Jul";"Aug";/
    "Sep";"Oct";/
    "Nov";"Dec"
mon "January";/
    "February";/
    "March";/
    "April";/
    "May";/
    "June";/
    "July";/
    "August";/
    "September";/
    "October";/
    "November";/
    "December"
% Appropriate date and time representation (%c)
d_t_fmt "%a %d %b %Y %r %Z"
%
% Appropriate date representation (%x)
d_fmt   "%m//%d//%Y"
%
% Appropriate time representation (%X)
t_fmt   "%r"
%
% Appropriate AM/PM time representation (%r)
t_fmt_ampm "%I:%M:%S %p"
%
% Strings for AM/PM
%
am_pm   "AM";"PM"
date_fmt "%F %Z"
END LC_TIME

Please let me know what I can do in order to get the expected format.

NOTE: I need to execute just date command without any formatting options.

Answer 1

In LC_TIME, there seems to be a separate way to specify the format for the date(1) command. This can be seen in 16.04's en_US locale definition:

% Appropriate date representation (date(1))   "%a %b %e %H:%M:%S %Z %Y"
date_fmt    "<U0025><U0061><U0020><U0025><U0062><U0020><U0025><U0065>/
<U0020><U0025><U0048><U003A><U0025><U004D><U003A><U0025><U0053><U0020>/
<U0025><U005A><U0020><U0025><U0059>"
END LC_TIME

These lines are missing from 17.04 and newer's en_US locale definition, but is still present in the C and POSIX locale files (so date maybe using these as a fallback).

If I do edit the en_US locale to add a date_fmt setting before END LC_TIME and update the locales, it works fine:

# grep date_fmt /usr/share/i18n/locales/en_US
date_fmt "%F %Z"
# env LC_TIME=en_US.UTF-8 date
2018-08-13 JST
# date
Mon Aug 13 15:25:14 JST 2018
# sed -i 's/date_fmt.*/date_fmt "%Y"/' /usr/share/i18n/locales/en_US
# locale-gen en_US.UTF-8
Generating locales (this might take a while)...
  en_US.UTF-8... done
Generation complete.
# env LC_TIME=en_US.UTF-8 date
2018