Steps to reproduce:

Set for any service downtime from:

Sunday 00:00 toSunday 12:30

Result:
Downtime is not set (without influence on service SLA)

Expected:
Planned downtime from 00:00 till 12:30

BUT

When you set downtime from:

Sunday 00:01 to Sunday 12:30 all warks well.

I suppose the same was in 3.4.x versions.