Date, Local Transaction
What it is
MMDD at the point of transaction, local time. Combined with DE 12 for the local timestamp.
DE 13 on the wire
Bitmap position
DE 13 is bit 13 of the primary bitmap: byte 2, mask 0x08. A primary bitmap with only DE 13 present reads 0008000000000000 in hex.
Length
DE 13 is fixed length: always 4 digits, with no length prefix. A parser reads exactly that many and moves on, so shorter values are right-aligned and padded with leading zeros.
Size in the message
At most 4 bytes when the message travels as text (ASCII or EBCDIC), or 2 bytes in binary dialects that pack digits as BCD.
Format n
Format n means numeric: the digits 0–9 only.
Specification reference
Defined in ISO 8583:1987 (base). Card networks publish their own dialect tables (Visa BASE I / V.I.P., Mastercard CIS) that refine formats, lengths, and — for private-use elements — the entire layout. Always confirm against the interface specification you implement.