diff --git a/test/plugin/duration.test.js b/test/plugin/duration.test.js
index 15e60b92..d248ee33 100644
--- a/test/plugin/duration.test.js
+++ b/test/plugin/duration.test.js
@@ -201,7 +201,7 @@ describe('Years', () => {
 describe('prettyUnit', () => {
   const d = dayjs.duration(2, 's')
   expect(d.toISOString()).toBe('PT2S')
-  expect(d.as('Second')).toBe(2)
+  expect(d.as('seconds')).toBe(2)
   expect(d.get('s')).toBe(2)
   expect(dayjs.duration({
     M: 12,
diff --git a/types/index.d.ts b/types/index.d.ts
index 46c2f933..22efc9eb 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -13,11 +13,11 @@ declare namespace dayjs {
 
   export type OptionType = { locale?: string, format?: string, utc?: boolean } | string | string[]
 
-  type UnitTypeShort = 'd' | 'M' | 'y' | 'h' | 'm' | 's' | 'ms'
+  export type UnitTypeShort = 'd' | 'M' | 'y' | 'h' | 'm' | 's' | 'ms'
 
-  type UnitTypeLong = 'millisecond' | 'second' | 'minute' | 'hour' | 'day' | 'month' | 'year' | 'date'
+  export type UnitTypeLong = 'millisecond' | 'second' | 'minute' | 'hour' | 'day' | 'month' | 'year' | 'date'
 
-  type UnitTypeLongPlural = 'milliseconds' | 'seconds' | 'minutes' | 'hours' | 'days' | 'months' | 'years' | 'dates'
+  export type UnitTypeLongPlural = 'milliseconds' | 'seconds' | 'minutes' | 'hours' | 'days' | 'months' | 'years' | 'dates'
   
   export type UnitType = UnitTypeLong | UnitTypeLongPlural | UnitTypeShort;
 
diff --git a/types/plugin/duration.d.ts b/types/plugin/duration.d.ts
index 3695eeaa..a5b197af 100644
--- a/types/plugin/duration.d.ts
+++ b/types/plugin/duration.d.ts
@@ -1,15 +1,22 @@
 import { PluginFunc } from 'dayjs'
+import { OpUnitType, UnitTypeLongPlural } from "../index";
 
 declare const plugin: PluginFunc
 export as namespace plugin;
 export = plugin
 
 declare namespace plugin {
-  type DurationInputType = string | number | object
-  type DurationAddType = number | object | Duration
+  type DurationUnitsObjectType = Partial<{
+    [unit in Exclude<UnitTypeLongPlural, "dates"> | "weeks"]: number
+  }>;
+  type DurationUnitType = Exclude<OpUnitType, "date" | "dates">
+  type CreateDurationType = 
+    ((units: DurationUnitsObjectType) => Duration)
+    & ((time: number, unit?: DurationUnitType) => Duration)
+    & ((ISO_8601: string) => Duration)
 
   interface Duration {
-    new (input: DurationInputType, unit?: string, locale?: string): Duration
+    new (input: string | number | object, unit?: string, locale?: string): Duration
 
     clone(): Duration
 
@@ -39,13 +46,13 @@ declare namespace plugin {
     years(): number
     asYears(): number
 
-    as(unit: string): number
+    as(unit: DurationUnitType): number
 
-    get(unit: string): number
+    get(unit: DurationUnitType): number
 
-    add(input: DurationAddType, unit? : string): Duration
-
-    subtract(input: DurationAddType, unit? : string): Duration
+    add: CreateDurationType;
+    
+    subtract: CreateDurationType
 
     toJSON(): string
 
@@ -59,10 +66,13 @@ declare namespace plugin {
 
 declare module 'dayjs' {
   interface Dayjs {
-    add(value: plugin.Duration): Dayjs
-    subtract(value: plugin.Duration): Dayjs
+    add(duration: plugin.Duration): Dayjs
+    subtract(duration: plugin.Duration): Dayjs
   }
 
-  export function duration(input?: plugin.DurationInputType , unit?: string): plugin.Duration
+  /**
+   * @param time If unit is not present, time treated as number of milliseconds
+   */
+  export const duration: plugin.CreateDurationType;
   export function isDuration(d: any): d is plugin.Duration
 }