Mean Time is a modern European invention, all Indian time systems were based on true motions. Hence, one AHORAATRA (day + night) is not exactly equal to 24 hours, but equal to the time from Sunrise to next Sunrise, and this duration is equal to 60 nadis / nadikas / ghatikas / ghatis / dandas (and each nadikas having 60 subs called vinadis or pala, each pala being equal to 60 vipala ; Amarkosha provides a detailed list of larger as well as smaller units of time).
But when we have to deal with Dina-maana and Raatri-maana (durations of day and night respectively) and divide them into many parts, we have to reckon the duration from Sunrise to Sunset and Sunset to Sunrise respectively, which are rarely equal to 12 hours each.
I had to face such problems in software designing, e.g., in lagna formulation when 24-hour ahoratra resulted in an insurmountable gap just before the Sunrise and therefore I had to follow the traditional definition of ahoratra as mentioned above.
Traditional Indian system is more natural, systematic and scientific.