Linux date command adds 10 hours to every datetime automatically – Debian

The GNU code for parsing dates (parse_datetime2) has this table embedded within it that provides the reason why you’re getting the incorrect time (trimmed for conciseness):

/* Military time zone table.

    Note 'T' is a special case, as it is used as the separator in ISO
          8601 date and time of day representation. */

static table const military_table[] =
{
    { "A", tZONE, -HOUR ( 1) },
    { "B", tZONE, -HOUR ( 2) },
    { "C", tZONE, -HOUR ( 3) },
              :  :  :
    { "Y", tZONE,  HOUR (12) },
    { "Z", tZONE,  HOUR ( 0) },
    { NULL, 0, 0 }
};

And, indeed, if you run date in debug mode, you can see what’s happening (output trimmed for conciseness):

pax> for i in {A..I} {K..Z} ; do
...>     date -d $i --debug 2>&1 | awk -vZ=$i '/parsed zone/{print "  "Z":"$5}'
...> done
  A:UTC-01
  B:UTC-02
    :  :
  T:UTC+07
    :  :
  Z:UTC+00

Most letters apply a timezone offset based on military timezones, hence the T that would normally be used to separate date and time in ISO format is being handled differently here, as specifying UTC+7, explaining the ten-hour difference between that and your server at UTC-3.

The bottom line is that you need to use an input format that’s acceptable to date. That’s as simple as (in this case) replacing the T with a space, as per the following transcript:

pax:~> ### First, the problematic one.
pax:~> date +"%Y/%m/%d %H:%M" -d "20200819T1234" --debug
date: parsed number part: (Y-M-D) 2020-08-19
date: parsed zone part: UTC+07
date: parsed number part: 12:34:00
date: input timezone: parsed date/time string (+07)
date: using specified time as starting value: '12:34:00'
date: starting date/time: '(Y-M-D) 2020-08-19 12:34:00 TZ=+07'
date: '(Y-M-D) 2020-08-19 12:34:00 TZ=+07' = 1597815240 epoch-seconds
date: timezone: system default
date: final: 1597815240.000000000 (epoch-seconds)
date: final: (Y-M-D) 2020-08-19 05:34:00 (UTC)
date: final: (Y-M-D) 2020-08-19 13:34:00 (UTC+08)
2020/08/19 13:34

pax:~> ### Then, the correct one.
pax:~> date +"%Y/%m/%d %H:%M" -d "20200819 1234" --debug
date: parsed number part: (Y-M-D) 2020-08-19
date: parsed number part: 12:34:00
date: input timezone: system default
date: using specified time as starting value: '12:34:00'
date: starting date/time: '(Y-M-D) 2020-08-19 12:34:00'
date: '(Y-M-D) 2020-08-19 12:34:00' = 1597811640 epoch-seconds
date: timezone: system default
date: final: 1597811640.000000000 (epoch-seconds)
date: final: (Y-M-D) 2020-08-19 04:34:00 (UTC)
date: final: (Y-M-D) 2020-08-19 12:34:00 (UTC+08)
2020/08/19 12:34