Mid-year HSA contribution eligibility

I have a question as someone new to HSA and a high-deductible plan (HDHP). I had a qualifying life event and enrolled in a HDHP through my employer today (9/3/2024), with the effective date of the health insurance being on 9/1.

I plan on contributing to an HSA outside of payroll and am not sure how many months I can contribute for. For this calendar year (2024), assuming that I have the HDHP from now until the end of the year, am I eligible to contribute for the months Sept-Dec, or only Oct-Dec? I ask because my employer's HR software had said that my HSA contribution would start on 10/01.

I did elect to contribute to a limited purpose FSA through my employer and have a similar question if anyone knows. My benefits software tells me that the FSA is effective on 10/01. Does this mean that FSA-eligible expenses incurred in September would NOT be reimbursable? Or does the "effective on" date for the FSA only indicate the start of payroll contributions?

submitted by /u/walkingoxymoron
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