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If you're looking for a simple, but not perfect solution consider using time.RFC3339 constant. But also know that there are differences between ISO8601 which are too complex for this answer.

See https://ijmacd.github.io/rfc3339-iso8601/ for differences and also has a handy test file generator to show differences. There is also a good discussion on SO here What's the difference between ISO 8601 and RFC 3339 Date Formats?

package main
import (
    "time"
    "fmt"
func main(){
    fmt.Println(time.Now().Format(time.RFC3339))
golang Time.Format
                This should be the accepted answer. The others may work, but they would certainly confuse someone who was unfamiliar with the code.
– Shadoninja
                Oct 24, 2017 at 17:05
                ISO 8601 and RFC3339 are not technically the same thing. stackoverflow.com/questions/522251/…
– 425nesp
                Dec 9, 2017 at 6:33
                From what I could tell, RFC3339 is a stricter version of ISO 8601. So it is probably safe to use the RFC format if a system expects the ISO.
– Noah Huppert
                Dec 28, 2017 at 8:17
                Note: It appears that converting to UTC first is not necessary: play.golang.org/p/UDrkigOfgIc
– lukeic
                Nov 9, 2018 at 2:04
                @lukeic that's because golang playground is working in UTC timezone. Try to run on your local machine.
– Flexoid
                Dec 11, 2018 at 11:24
                Note that if you include UTC(), the formatted string will always  UTC offset +0000. To preserve the local time offset, omit UTC().
– nishanthshanmugham
                Feb 16 at 16:49
first I found the time.RFCxxx that was the closest to my target
I copied its value
I fiddled with it until I found the expected result
which is
2006-01-02T15:04:05.999Z
                The documentation for the time.Layout constants explains the format specifiers: pkg.go.dev/time#pkg-constants
– nishanthshanmugham
                Feb 16 at 16:51
ISO8601 allows for variable levels of granularity.  You can have just a year, year+month, year+month+day, add a time portion, and optionally have a timezone portion.  Go's built-in time parsing, however, requires you to know ahead-of-time which parts will be included.
The github.com/btubbs/datetime library provides a more flexible parser that can handle all the commonly used ISO8601 formats.  See https://github.com/btubbs/datetime
Disclosure: I wrote that library.
                "UnmarshalJSON methods support easy parsing of ISO 8601 timestamps from external systems"
– lsl
                Jan 5, 2021 at 11:46
                Thanks a lot! Works as advertised, parses "20220201T204925Z"  which is super useful when you cant have colon in the format. I wish there was a datetime.Format(...) function as well.
– Steven Varga
                Feb 1, 2022 at 22:02
Replacing the sign in the format with a Z triggers the ISO 8601 behavior. Which is exactly time.RFC3339. If you are wanting the string output to end in 'Z' what you need to do is convert to the UTC zone.
package main    
import (
    "fmt"
    "time"
func main() {
    fmt.Println(time.Now().UTC().Format("2006-01-02T15:04:05Z07:00"))
// this is the same format used by RFC3339. just a note on why. 
                I'm sure that it does print the correct time stamp. This is the same format used in time.Time. See golang.org/src/time/format.go?s=15423:15465#L78
– dustinevan
                Mar 5, 2018 at 18:45
                Thanks a lot!  I could format to ISO8601 basic (no colons) : fmt.Println(time.Now().UTC().Format("20060102T150405Z")) in fact it works the other directions as well: tm, err := time.Parse("20060102T150405Z", "20220201T204925Z")
– Steven Varga
                Feb 1, 2022 at 22:06
        
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