So assuming the "20 years" figure to be accurate, and not either rounded or just plain wrong (like the age of the
Enterprise in TSFS):
- April's Enterprise-α was commissioned sometime between 2233 (earlier would be a violation of Prime canon) and 2240.
- April became captain sometime between commissioning and when he was lost - earlier is "better", to give him more time to build up enough of a legend where Kirk recognizes the name.
- April was lost between 2238 and 2240.
- Countdown to Darkness takes place sometime between 2258 and 2260.
I hope we find out more about April's
Enterprise, how it came to be, and how it's different from his
Enterprise as depicted (apocryphally) in Diane Carey's books...