The first day on the billing statement covering services rendered to the beneficiary (a.k.a. 'Statement Covers From Date').
The "from" date on the claim may not always represent the first date of services, particularly for Home Health care. To obtain the date corresponding with the onset of services (or admission date) use the admission date from the claim (variable called CLM_ADMSN_DT for IP, SNF and HH.
For Part B Non-institutional (Carrier and DME) services, this variable corresponds with the earliest of any of the line-item level dates (i.e., in the Line File, it is the first CLM_FROM_DT for any line on the claim). It is almost always the same as the CLM_THRU_DT; exception is for DME claims - where some services are billed in advance.
Source: Medicare Advantage Organizations (MAOs)