That was a great start @mdevaney ,
The only issue was the time from 00:00 to 05:45.
I changed the formula a little, and it works great now.
With(
{myTime: Time(HourValue1.Selected.Value,MinuteValue1.Selected.Value, 0)},
If(
myTime >= Time(00,00,0) And myTime < Time(5,45,00), "N",
myTime >= Time(5,45,00) And myTime < Time(13,45,0), "O",
myTime >= Time(13,45,00) And myTime < Time(21,45,0), "M",
myTime >= Time(21,45,00), "N"
)
)
Thanks for the very quick reply!