fix: update customParseFormat plugin to parse 2-digit offset
fix #1205
**Describe the bug**
PostgreSQL is returning ```time with time zone``` fields in this ISO 8601 format: ```2020-12-01T20:00:00+00```. Note that it's using a 2-digit offset which is part of the ISO specification, along with the 4-digit colon-separated format (which Day.js parses fine).

Day.js doesn't parse this 2-digit format, it ignores the timezone if parsed normally, i.e. ```dayjs('2020-12-01T20:00:00+00')```, and if given a custom format like ```dayjs('2020-12-01T20:00:00+00', 'YYYY-MM-DD[T]HH:mm:ssZZ');``` it will return an ```Invalid date```.

Sandbox w/ test and control strings: https://codesandbox.io/s/festive-glitter-mh7bx

**Expected behavior**
I expect this format to be parsed correctly like the 4-digit, colon separated ISO 8601 timezone offset format ```+NN:NN```.

**Information**
 - Day.js Version: 1.9.6
 - OS: macOS
 - Browser: Chrome, Safari, Firefox
 - Time zone: GMT+1

Day.js doesn't parse ISO 8601 timezone offset +NN
diff --git a/src/plugin/customParseFormat/index.js b/src/plugin/customParseFormat/index.js
index 940bb43a5..f2eabb2b7 100644
--- a/src/plugin/customParseFormat/index.js
+++ b/src/plugin/customParseFormat/index.js
@@ -8,14 +8,15 @@ const match3 = /\d{3}/ // 000 - 999
 const match4 = /\d{4}/ // 0000 - 9999
 const match1to2 = /\d\d?/ // 0 - 99
 const matchSigned = /[+-]?\d+/ // -inf - inf
-const matchOffset = /[+-]\d\d:?\d\d/ // +00:00 -00:00 +0000 or -0000
+const matchOffset = /[+-]\d\d:?(\d\d)?/ // +00:00 -00:00 +0000 or -0000 +00
 const matchWord = /\d*[^\s\d-:/()]+/ // Word
 
 let locale
 
 function offsetFromString(string) {
+  if (!string) return 0
   const parts = string.match(/([+-]|\d\d)/g)
-  const minutes = +(parts[1] * 60) + +parts[2]
+  const minutes = +(parts[1] * 60) + (+parts[2] || 0)
   return minutes === 0 ? 0 : parts[0] === '+' ? -minutes : minutes // eslint-disable-line no-nested-ternary
 }
 

diff --git a/test/plugin/customParseFormat.test.js b/test/plugin/customParseFormat.test.js
index 03b446a63..ef8bfd625 100644
--- a/test/plugin/customParseFormat.test.js
+++ b/test/plugin/customParseFormat.test.js
@@ -101,6 +101,22 @@ it('timezone with no hour', () => {
   expect(dayjs(input, format).valueOf()).toBe(moment(input, format).valueOf())
 })
 
+describe('Timezone Offset', () => {
+  it('timezone with 2-digit offset', () => {
+    const input = '2020-12-01T20:00:00+09'
+    const format = 'YYYY-MM-DD[T]HH:mm:ssZZ'
+    const result = dayjs(input, format)
+    expect(result.valueOf()).toBe(moment(input, format).valueOf())
+    expect(result.valueOf()).toBe(1606820400000)
+  })
+  it('no timezone format token should parse in local time', () => {
+    const input = '2020-12-01T20:00:00+01:00'
+    const format = 'YYYY-MM-DD[T]HH:mm:ss'
+    const result = dayjs(input, format)
+    expect(result.valueOf()).toBe(moment(input, format).valueOf())
+  })
+})
+
 it('parse hh:mm', () => {
   const input = '12:00'
   const format = 'hh:mm'

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"fix": "PASS"}, "test/timezone.test.js:Utc Offset": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/utc-utcOffset.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/sr-cyrl.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/br.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isMoment.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isYesterday.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/pl.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/toObject.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/sr.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isSameOrAfter.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/weekOfYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/comparison.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/et.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/hu.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/calendar.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isTomorrow.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:UTC and utcOffset": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/arraySupport.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isToday.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/hr.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/badMutable.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/duration.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isoWeek.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/minMax.test.js": {"run": "PASS", "test": "PASS", "fix": 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{"test/plugin/customParseFormat.test.js": {"run": "PASS", "test": "FAIL", "fix": "PASS"}}
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