If you talk about an event's anniversary, like "it's our anniversary" to mean when you got married, or "it's my anniversary at the company" to mean a year after when you were hired, then the listener will assume it's a one-year interval.
But if you add extra words to indicate the interval explicitly, such as "it's our 6 month anniversary" to mean 6 months after you met, or after you got married, then then the listener will know it's for less than one full year.
So by default anniversary means a yearly interval (and this is the official usage) but in speaking, people sometimes want to celebrate a shorter interval, so they qualify the term with more adjectives, and then it is clear.
Note that discussing anniversaries that are shorter than a year is usually done by younger people, for whom dating someone for 5 whole months is a "long time" so if I overhear someone on the bus say "It's our 5 month anniversary!" then I will expect to look over and see someone who is 20, not someone who is 50.
Also, its is rare to combine years and months, for just this reason... Once you are at a year, your usually count by years. You don't hear "it's my 6 1/2 year anniversary".