FIRSTDATE / LASTDATE — DAX function reference

First/last date in context.

Syntax

dax

FIRSTDATE(<dates>)    LASTDATE(<dates>)

dax

LASTDATE('Date'[Date])

→ A date

Often used as the opening side of an opening/closing balance pair.

dax

CALCULATE(SUM(Sales[Amount]), FIRSTDATE('Date'[Date]))

→ Sales on the earliest date currently in context

The standard pattern for non-additive measures like inventory or headcount.

dax

CALCULATE([Total Inventory], LASTDATE('Date'[Date]))

→ Closing/point-in-time balance

FIRSTDATE/LASTDATE return a one-row table of the min/max date in context, not a scalar — that’s why they’re typically wrapped in CALCULATE, though DAX implicitly coerces a single-row, single-column table to a scalar in most contexts.
They differ from plain MAX('Date'[Date]): FIRSTDATE/LASTDATE are the time-intelligence-aware building blocks other functions like SAMEPERIODLASTYEAR use internally and behave predictably with non-contiguous selections, while MAX is a simpler scalar aggregation that’s fine (and faster) for most report measures.

From Time intelligence