Help with Variables and Child/Parent

Guys, i’ve a doubt about using variables in Grafana.

I’m using grafana to make a query (In Mysql) to generate a report monthly. On this query, i do use some variables “CONVERT_TZ(from_unixtime(E.clock), @@session.time_zone, ‘-03:00’) >= ‘[[ano]]-’’[[mes]]’’-01 12:00:00’”. In that case, the “ano” (year) i did created a custom variable with some years (2018,2019,2020, etc). Variable “mes” (Month), i did a custom variable that count from 01 to 12.

My doubt is, i wanna make another variable that hard coded the days, for example, if a choose 1 (January) in month variable, the variable “Day” change to “31”, if i choose 2 (February), the Day change 28, etc.

Can we make these kind of variable in Grafana? If so, you can help me?