diff --git a/src/pkg/flag/flag.go b/src/pkg/flag/flag.go
index cd2a165be1..5797cc79d4 100644
--- a/src/pkg/flag/flag.go
+++ b/src/pkg/flag/flag.go
@@ -628,18 +628,21 @@ func Float64(name string, value float64, usage string) *float64 {
 
 // DurationVar defines a time.Duration flag with specified name, default value, and usage string.
 // The argument p points to a time.Duration variable in which to store the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
 func (f *FlagSet) DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
 	f.Var(newDurationValue(value, p), name, usage)
 }
 
 // DurationVar defines a time.Duration flag with specified name, default value, and usage string.
 // The argument p points to a time.Duration variable in which to store the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
 func DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
 	CommandLine.Var(newDurationValue(value, p), name, usage)
 }
 
 // Duration defines a time.Duration flag with specified name, default value, and usage string.
 // The return value is the address of a time.Duration variable that stores the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
 func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time.Duration {
 	p := new(time.Duration)
 	f.DurationVar(p, name, value, usage)
@@ -648,6 +651,7 @@ func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time
 
 // Duration defines a time.Duration flag with specified name, default value, and usage string.
 // The return value is the address of a time.Duration variable that stores the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
 func Duration(name string, value time.Duration, usage string) *time.Duration {
 	return CommandLine.Duration(name, value, usage)
 }