Finer Expiry Control

[Goirad]

Hello,
I find that I often need to create expired certificates for testing related validation and it would be extremely helpful if the --expires option was extended to allow passing minutes and even seconds.

[mbyczkowski]

I think it'd probably be fine to have a flag that takes expiry in simple duration format 1d10h2m5s

[Goirad]

Or even better would be the ability to offset the start date backwards in time by a similar mechanism
ie --offset -1y --expires 1m to create a certificate valid starting a year ago for a period of 1 month