There are two ways to do it. The way many schools do it is to say:
"It's 22 minutes until 7", which would be "Son las siete menos 22" (Literally It's 7 minus 22)
Honestly though, any Spanish speaking person will understand you if you say "Son las séis (con) treinta y ocho". The con is optional and you really don't need to say it.
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