Does January always have 31 days?

YES!!!!

Remember the old verse, on the days of each month.

' Thirty(30) days hath(has) September,

April, June and November.

All the rest have thirty one(31),

Excepting February alone,

Which has twenty eight(28) days each year,

And twenty nine(29) in a leap year. ' .

Bracketed terms are there for clarity only.

So as January is amongst 'All the rest', then it has thirty one days(31).