"adjacent" is the key detail to look at carefully. If you can be confident that the two records will always be "adjacent" in any found set or set of related records in a portal, GetNthRecord can be used to access data in the previous record:
Let ( r = Get ( RecordNumber ) ; If ( r > 1 ; GetNthRecord ( Date ; r - 1 ) ; 0 ) )
All of these functions can be looked up in FileMaker help if they are unfamiliar.
For faster calculations and less chance of hitting a "recursion limit", I'd set up a number field with this expression as an auto-enter calculation to copy this data from the previous record at the time that it is created. Otherwise, you can see long delays waiting for large found sets to update and you can see records lower down int he found set fail to calculate--especially when using FM GO on an iOS device. This also avoids the need for having the records perfectly adjacent at all times as long as they are adjacent when a new record is added to the table. The key draw back to such an approach is that if you change the date field in record 1, this calculation will not automatically update in record 2.
There are also ways to use a relationship or the ExecuteSQL() function (FileMaker 12 or newer) to access the data from a previous record.
Will this work on sorted records? I may make 3 entries to record loan disbursements on the same day but for different loan customers. Then the table is sorted by customer. The meaningful calculation is the difference in day between disbursements for THAT CUSTOMER; unsorted, those disbursements may be table records 101 and 104.
That's why "adjacent" is the key detail.
If the new record that you create is adjacent to the previous record, such as performing a find for all such records for a particular client, sorting and then creating the new record, then yes, this will work.
But if you want to create several new records without first pulling up the other records for that client, then you'd need to use another method such as a self join relationship that uses a specified sort order in the relationship to refer to the immediately previous disbursement record from the same client even if the record is not present in the found set.