figure out how many days it's been since your calendar started (earth time) convert that using the length of your calendar year in earth years
say it's been 500 earth years and your calendar is 300 days long multiply 500 * (365.25/300) to get how many of your calendar years
convert the remaining decimal into days by multiplying by how long your year is in your days
say you have 100 days and it has been idk something.3 years, then it's .3 * 100, 30 days
figure out what that number means in your calendar from there
thanks ahooWait, why thank me?