Submitted by: @mrotteveel

Implement SQL standard FORMAT clause for CAST between string types and datetime types, to allow custom formatting of datetime values and conversion from string values with a specific format to datetime values.

"""
<cast specification> ::=
CAST <left paren>
<cast operand> AS <cast target>
[ FORMAT <cast template> ]
<right paren>

<cast operand> ::=
<value expression>
| <implicitly typed value specification>

<cast target> ::=
<domain name>
| <data type>

<cast template> ::=
<character string literal>
"""

Where <cast template> follows the rules of Subclause 9.42, "Converting a datetime to a formatted character string" or Subclause 9.43, "Converting a formatted character string to a datetime". Specific syntax rules defined in Subclause 9.44, "Datetime templates":

"""
<datetime template> ::=
{ <datetime template part> }...

<datetime template part> ::=
<datetime template field>
| <datetime template delimiter>

<datetime template field> ::=
<datetime template year>
| <datetime template rounded year>
| <datetime template month>
| <datetime template day of month>
| <datetime template day of year>
| <datetime template 12-hour>
| <datetime template 24-hour>
| <datetime template minute>
| <datetime template second of minute>
| <datetime template second of day>
| <datetime template fraction>
| <datetime template am/pm>
| <datetime template time zone hour>
| <datetime template time zone minute>

<datetime template delimiter> ::=
<minus sign>
| <period>
| <solidus>
| <comma>
| <apostrophe>
| <semicolon>
| <colon>
| <space>

<datetime template year> ::=
YYYY | YYY | YY | Y

<datetime template rounded year> ::=
RRRR | RR

<datetime template month> ::=
MM

<datetime template day of month> ::=
DD

<datetime template day of year> ::=
DDD

<datetime template 12-hour> ::=
HH | HH12

<datetime template 24-hour> ::=
HH24

<datetime template minute> ::=
MI

<datetime template second of minute> ::=
SS

<datetime template second of day> ::=
SSSSS

<datetime template fraction> ::=
FF1 | FF2 | FF3 | FF4 | FF5 | FF6 | FF7 | FF8 | FF9

<datetime template am/pm> ::=
A.M. | P.M.

<datetime template time zone hour> ::=
TZH

<datetime template time zone minute> ::=
TZM
"""