[Spaceships] Voyage Time Calculation with Different Drives

[nondescript handle]

This may be glaringly obvious, but could someone please explain to me how to calculate interplanetary (or planet-jumppoint-planet) travel time, when the spaceship uses two different STL reaction drives, especially if they accelerate (or decelerate) simultaneously during some (but not necessarily all) of the time?

Maybe one example to clarify what I mean:

The "Donna Jackson-Morley" is a TL 10 and SM +10 deep space container ship on its way from the port station in Earth orbit to Ceres High Port. The GM rules that the distance is 3 AU.

The Donna has two magsail systems, one fusion rocket system, and one reaction mass tank filled with 20 mps worth of water. The average distance from the sun during the trip is roughly 2 AU, so the magsails can provide an acceleration of 0.0005 G.

After breaking earth's orbit the Donna powers up her magsails and ignites her fusion rocket, and for the first 30 hours she accelerates with a combined 0.0155 G.
After that she turns off the fusion rocket and accelerates only with her magsails until the midpoint at which she turns around and starts decelerating with the magsails.
At the last 30 hours of voyage she decelerates with both the fusion rocket and the magsails.

What is the duration of this trip, and, more importantly, how do I calculate that time?

Thank you!

Yes, this mode of travel is inspired by Nathan Lowell's "Golden Age of the Solar Clipper" series ;-)