[1] IMAP で使われる日付形式は、同じ電子メイル関係で使われる形式である RFC2822の日付形式にもやや似ていますが、微妙に異なり、互換性はありません。

[5] RFC 3501 - INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1 (2018-02-01 03:51:41 +09:00) https://tools.ietf.org/html/rfc3501#page-85
[6] RFC 2060 - Internet Message Access Protocol - Version 4rev1 (2018-01-28 19:02:32 +09:00) https://tools.ietf.org/html/rfc2060#page-68

[2] RFC 3501 ABNF 抜粋 :

date = date-text / DQUOTE date-text DQUOTE
date-day = 1*2DIGIT ; Day of month
date-day-fixed = (SP DIGIT) / 2DIGIT ; Fixed-format version of date-day
date-month = "Jan" / "Feb" / "Mar" / "Apr" / "May" / "Jun" / "Jul" / "Aug" / "Sep" / "Oct" / "Nov" / "Dec"
date-text = date-day "-" date-month "-" date-year
date-year = 4DIGIT
date-time = DQUOTE date-day-fixed "-" date-month "-" date-year SP time SP zone DQUOTE
time = 2DIGIT ":" 2DIGIT ":" 2DIGIT ; Hours minutes seconds

zone = ("+" / "-") 4DIGIT

          ; Signed four-digit value of hhmm representing
          ; hours and minutes east of Greenwich (that is,
          ; the amount that the given time differs from
          ; Universal Time).  Subtracting the timezone
          ; from the given time will give the UT form.
          ; The Universal Time zone is "+0000".

[4] date-text は必ずしも固定長ではなく、 date-time は必ず固定長になります。どちらの定義が使われるかは場面によります。

[3] 例 :

17-Jul-1996 02:44:25 -0700

[12] 時間帯についてはインターネットメールの時間帯表記も参照。

[7]

RFC 1064 - Interactive Mail Access Protocol: Version 2 (2018-02-04 11:06:30 +09:00) https://tools.ietf.org/html/rfc1064#page-24

date ::= string in form "dd-mmm-yy hh:mm:ss-zzz"

[8]

RFC 1176 - Interactive Mail Access Protocol: Version 2 (2018-01-21 21:01:02 +09:00) https://tools.ietf.org/html/rfc1176#page-26

date ::= string in form "dd-mmm-yy hh:mm:ss-zzz"

[9]

RFC 1176 - Interactive Mail Access Protocol: Version 2 (2018-01-21 21:01:02 +09:00) https://tools.ietf.org/html/rfc1176#page-22

InternalDate " 9-Jun-88 12:55:44 PDT"

[10]

RFC 1203 - Interactive Mail Access Protocol: Version 3 (2018-01-23 00:10:54 +09:00) https://tools.ietf.org/html/rfc1203#page-38

date ::= string in form "dd-mmm-yy hh:mm:ss-zzz"

[11] (1992-12-12 09:00:00 +09:00) https://www.mirrorservice.org/sites/ftp.cac.washington.edu/mail/old/IMAP2bis.TXT

date ::= date_new / date_old
date_new ::= string in form "dd-mmm-yyyy hh:mm:ss -zzzz"
date_old ::= string in form "dd-mmm-yy hh:mm:ss-zzz"
In date_new, the year is now a 4-digit year, and the timezone is a
signed four-digit value of hhmm representing hours and minutes west
of Greenwich (that is, the amount that the given time differs from
Universal Time). Subtracting the timezone from the given time will
give the UT form.
Universal Time is expressed as "+0000".
date_old is hereby declared obsolete, and its usage in new software
is STRONGLY discouraged. However, client implementations MUST
recognize and properly parse it.
RFC-1176 documents the argument to the SEARCH criteria BEFORE, ON, and
SINCE as being either "date" in the text and "string" in the BNF.
These SEARCH criteria arguments are hereby amended to be "day", with
the following syntax:
day ::= day_new / day_old
day_new ::= string in form "dd-mmm-yyyy"
day_old ::= string in form "dd-mmm-yy"
day_old is hereby declared obsolete, and its usage in new software
is STRONGLY discouraged. However, server implementations MUST
recognize and properly parse it.