how to generate new root CA with given start/end dates

I'd like to generate a new root CA with a not_before date of my choice.

My basic, working setup is:
```json
{
"CN": "My root CA",
"key": {
"algo": "ecdsa",
"size": 384
},
"ca": {
"expiry": "87600h",
"pathlen": 1
},
"names": [{
"C": "my country",
"ST": "my state",
"L": "my location",
"O": "my org",
"OU": "my unit"
}]
}
```

...and I generate the CA with:
```
cfssl gencert -initca root_ca.json | cfssljson -bare root_ca
```

I have tried adding a `config.json` like this:
```
{
"signing": {
"default": {
"not_before": "2018-07-01T00:00:00Z",
"not_after": "2019-07-01T00:00:00Z"
}
}
}
```
or trying the same inside a signing / default block, or similar things.

How could I explicitly set `Not Before` and `Not After` (or a `Not Before` and an expire time) on a Root CA?

I'm not a Go coder but looking at `func Update()` in [initca/initca.go](https://github.com/cloudflare/cfssl/blob/1.3.2/initca/initca.go#L233) I fear that it is hardcoded to the system's clock...