A bit of fiddling around with the season ticket calculator yields:

Zone 1-4 1 month £151.30, 1 month 1 day £156.50, 1 month 2 days £161.60, 1 month 3 days £166.70, 1 month 4 days £171.80, 1 month 5 days £176.60, 1 month 6 days £181.70, 1 month 7 days £186.80, 1 month 8 days £191.90, 1 month 9 days £197.00, 1 month 10 days £201.80, ... 1 month 29 days £297.90, 1 month 30 days £302.60, 2 months also £302.60.

The increase is £5.20 for the first day, then £5.10 for days 2-4, £4.80 for day 5, and then £5.10 for the next 4 days and £4.80 for every 5th day, repeating.

To me that tends to suggest it's based on a 30-day month but there is some element of rounding involved. Every 5-day period costs the same at one-sixth of the monthly cost, then it's divided by 5 and rounded up in the first four days and down in the last day to get the daily costs.

There is also probably a painfully small saving to be made in February