What is the impact of leap year on the "30E/360" Day Count Convention?

Leap year has no impact on the 30E/360 Day Count Convention, as long as the Accrual Frequency is Monthly, Quarterly, Semiannually, or Annually and Accrual Period boundaries are not falling on End of Month in February.

Here is an example of Norwegian government bond security.

ISIN:                   NO0013713123
Issuer:                 Norwegian government
Interest rate:          4.125%
Interest frequency:     Annually (1 time in a year)
Day Count Convention:   30E/360
Term:                   10 Years 
  Start Date:           2026-03-02
  Maturity Date:        2036-03-06

If you bought 1,000.00 NOK of this Swedish bond, you will get paid 41.25 NOK in each Accrual Year, including leap years. For example, the Accrual Year of [2027-03-02, 2028-03-02) contains a leap day of 2028-02-29. The Accrued Interest for this Accrual Year is still 41.25 NOK:

T1 = 2027-03-02: Starting date (inclusive)
T2 = 2028-03-02: Ending date (exclusive) of the Accrual Range 
T3 = 2028-03-02: Ending date (exclusive) of the Accrual Period  
T4 = 2028-03-02: Ending date (exclusive) of the Accrual Year

DiR(Y1,M1,D1,Y2,M2,D2) 
  = 360×(Y2-Y1) + 30×(M2-M1) + (Minimum(D2,30)-Minimum(D1,30))
  = 360×(2028-2027) + 30×(3-3) + (2-2)
  = 360

DiY(Y1,M1,D1,Y2,M2,D2) = 360

Day_Count_Factor(Y1,M1,D1,Y2,M2,D2)
  = DiR(Y1,M1,D1,Y2,M2,D2) / DiY(Y1,M1,D1,Y2,M2,D2)
  = 360/360 
  = 1

Accrued_Interest = Principal × Interest_Rate × Day_Count_Factor
  = 1,000.00 NOK × 4.125% × 1 
  = 41.25 NOK

References:

• Norwegian, Königreich 4,125% 26/36, deutsche-boerse.com.

⇒ "30E/360" - Daily Compound Interest

⇐ "30E/360" - Cross-Period Accrual

⇑ Day Count Convention - "30E/360"

⇑⇑ Day Count Conventions