Hi, I'm new in all this so sorry for newbie questions. I register Ltd. in the UK on January 28th this year, and I'm not sure when (my) fiscal year ends, on Jan 28, on Mar 31, April 5,6,... I suppose on Jan 31 2020?
In the UK, "the fiscal year" runs from 6th April to 5th April. However, if you have registered a Limited Company, it is your choice to set an appropriate Year End for your business to report. Many companies use the April-April fiscal year for convenience; others use calendar years; others use some other date for their own reason.
Choosing an appropriate Year End is something you should discuss with your accountant.
Hope that helps!